Enforced Technology Levels

This is a Speculative Fiction trope where, for whatever reason, there are certain rules about what kinds of technology can exist. The ways these rules are enforced vary, from the the relatively mundane (some kind of "technology" police) to fantastic (magic, nanotech, or the laws of nature themselves). What's consistent, though, is that these rules will effectively (or even explicity) lock a society into a certain technological level.

When magic is involved, this might delve into a Magic Versus Technology War (since areas where magic works may not allow technology to work, and vice versa). For a rather specific example (when it's played literally), see Medieval Stasis. Compare with Schizo-Tech, where differing levels of technology are all mashed up together. Compare Decade Dissonance, when this sort of things arises naturally, not due to any enforcement. See also You Are Not Ready, which might be the justification for keeping certain areas at a lower tech level.

Examples of Enforced Technology Levels include:

Fan Works

Some Harry Potter fanfiction postulates that modern technology is discouraged or outright banned in the Wizarding World. In some stories, this goes as far as saying that using a gun is considered a worse crime than using an Unforgiveable Curse.

Film

The film Men in Black features elements of this, as some of the alien cultures humanity has contact with are much more advanced than humans. The MIB organization prohibits certain advanced technologies on Earth, reasoning that humanity shouldn't be allowed to discover them until we're ready.
- It's stated that they slowly release some of the alien technology using shell companies and finance their operations using the patents. A lot of it is confiscated tech, although some of it is given by friendly aliens (like those tall guys in the picture).
One Dexter's Laboratory movie featured a dystopian future where Mandark rules, and keeps people in line by preventing them from using any form of science (even going so far as prohibiting the practice of using friction to start a fire).

Literature

Lady of Mazes by Karl Schroeder is set on a space station with varying technological levels enforced by ubiquitious nanotech. Different groups of people, united by common philosophies/religions, have chosen the level of tech they're comfortable with. For example, a society that values communing with nature and living in harmony might purposefully limit themselves to only Stone Age tech. If someone from a more advanced area goes into a "primitive" area, they will find that none of their high-tech gadgets work. The main conflict in the novel is due to a single group trying to enforce their high-tech Hive Mind on everyone.
- Another example from Schroeder is in his novel Ventus. The eponymous planet was terraformed by powerful AIs called "Winds" in order to be a paradise for their human masters. However, something went wrong, and the AIs no longer recognize humans. As soon as the colonists arrived their ships were shot down because the Winds see their technology as a threat to the fragile ecosystem of the planet. The survivors are forced to eke out a primitive existence on the planet, since anything more advanced than a plow is immediately destroyed by the Winds.
Terminal World by Alastair Reynolds features a giant megastructure known as "Spearpoint", a spiraling tower that is the last city on Earth. Within it, various technological levels are enforced by reality itself; the laws of nature seem to change between the levels. The higher up the tower, the more advanced technology becoming possible the higher up one goes. One of the lower levels is called "Steamtown", a higher up level is called the "Neon Heights", and even further up is "Circuit City". It's implied that people from the top of the tower, the "Celestial Zone", cannot even go to the lower levels, due to the advanced nanotech in their cells that starts to break down as soon as they go outside their zone.
In Vernor Vinge's Zones of Thought series, the physical laws of nature seem to vary depending on how far one is from the galactic core. Such that, the further you get away from the core, the more advanced technology is able to be. Earth is located in the "slow zone", where physics works as we currently understand it (i.e. faster-than-light travel is impossible, no such thing as anti-gravity, etc). Further out is called "The Beyond", where things like FTL travel and Artificial Intelligence become possible. Farthest is "The Transcend", a zone where magic and science lose any distinction and you have things like powerful AIs becoming akin to gods.
The short stories "Jury Duty" and "Appeals Court", both collaboratively written by Cory Doctorow and Charles Stross, are set in the aftermath of a Singularity where the remaining un-"raptured" population is closely guarded against marauding technology by a "technology court". Citizens are randomly signed up to serve on this court (as the namesake "Jury Duty" would imply), and their job is to evaluate the random bits of super advanced flotsam-and-jetsom that fall to Earth from the swirling mass of former-humans that has expanded into and taken over space. The technology court is tasked with determining what new technology's effect on society at large will be, and anything too dangerous or too advanced (that might cause another singularity in the remaining population) is destroyed.
In Harry Potter, magical spells can prevent certain technologies from functioning. On Hogwarts school grounds, things like guns, automobiles, or anything elecronic simply won't work. Also, it's against the rules to enchant high-tech items.
In the Old Kingdom series, technology or anything made with technology rapidly degrades into ruin in the Old Kingdom. There was one guy whose outsider pen pal always uses machine-made paper, turning every letter he sends into an annoying exercise in forensic science. This is because most technology fails in the presence of magic (this also means that the Perimeter Guards are armed with both guns and swords, because any magical creature that gets close enough will make their guns fail).
The Well World novels (two series, by Jack L Chalker) take place on an artificial planet divided into hexagonal territories. The creators of the Well World set up each hexagon to be home to and a test bed for a different prototype intelligent species, prior to transferring these new races to various uninhabited planets. Thus, each hexagon re-creates the environment to which the new-made species is intended to be moved, once it's been tested. As some of their intended destinations were lacking in metals or other materials needed for technological advancement, the hexagons where those planet's future inhabitants were tested had to be designed to prevent technology from working there: if the trial groups couldn't survive by low-tech methods, then they would need more tinkering before they could be shipped out to their new worlds.
- As the Well World's creators had the means to re-write the laws of physics as they saw fit, it was easy for them to invoke this trope on designated low-tech hexes, with tech-failure kicking in at whatever point seemed appropriate to the destination planet (Stone Age, Iron Age, Clockwork, etc).
  - Originally, there was no movement allowed between hexes, however since the Markovians left the barriers (mostly) went down, and a large part of the trade that developed between hexes involved moving the products of technology: a gun might not work in a low-tech hex, but a composite bow made from high-tech materials works just peachy, thank you.
The Church of God Awaiting in Safehold enforces Medieval Stasis through the Inquisition and the Proscriptions of Jwo-Jeng.
- It is also enforced, though the citizens of Safehold don't know it, by orbital platforms that, if they detect strong enough power sources, will unleash a kinetic bombardment capable of devastating a small continent.
In the George Lucas Throwback Space Opera novel Grand Central Arena, certain technologies just don't work in the Arena, including AI, nuclear reactors, and nanotech beyond certain limits.
On Gor the Priest-Kings will smite anyone they catch experimenting with "forbidden technology" such as firearms. However, they do allow experimentation in some areas, such as medicine, which has advanced to the point that the Goreans are basically indestructible to disease and age.
In the Weis/Hickman Starshield books, the laws of physics aren't constant, but regional. Here, Newtonian/Einsteinian physics apply, over here it's demons and magic, over there it's sorcery.
In the future Earth of the Council Wars, the omnipresent AI taking care of the planet, Mother, strictly controls how much energetic reactions can be used, with an upper limit that can't normally be breached, for public safety. When everything goes to hell, this means that firearms and explosives are impossible, and even most engines beyond very low-pressure steam ones. On the other hand, they've got several millennia of genetically-engineered crops and animals, previously-built supermaterials, and the odd item provided by the people who still have access to the Clarke-level tech.
A Russian novel has a colony on a planet whose star is unusually active and is constantly throwing up strong electromagnetic fields that interfere with most advanced technology. Only specially-shielded ships are used to travel to the colon. When a Middle Eastern nation takes over the colony, the Russian and German Empires send a joint fleet of their own to liberate the colony... using German World War Two tanks. And the German soldiers here are the friendliest you can find, which is saying something, considering that this is a Canadian colony.
A Russian duology by Aleksandr Mazin (Time for Change and The Morning of Judgment Day) feature a world in the near future where nature itself seems to have rebelled against advanced technology. After a series of seemingly random catastrophes, which were correlated with research into certain areas of science which some may find questionable, a global ban was placed on specific fields of science and an international agency was set up to keep tabs and stop any illegal research. At the end of the second novel, China outright ignores the ban and launches a manned mission to Mars (spaceflight is one of the banned areas). However, just as the ship is about to reach Mars, all Chinese-speaking people in the world who are watching the transmission are rendered mute. The protagonist's father points out parallels between this and the Tower of Babel (i.e. humans attempting to reach the Heavens) and postulates that, perhaps, humanity is meant to stay on Earth.
In Jerry Pournelle's early CoDominium stories, the Bureau of Technology puts all scientific and technological advancement under tight controls to prevent the creation of any devices that would threaten the stability of society.

Live-Action Television

In Star Trek, the Federation is prohibited from developing its own cloaking devices (which is like saying only one side can have submarines).
- A whole TNG episode is devoted to trying to cover up one such attempt at developing it.
- The game Star Trek Away Team features a modified Defiant-class ship whose holo-masking system tries to sidestep the letter of the treaty, if not the spirit. The USS Incursion can appear as any other ship, even sending out falsified transponder signals. Not true cloaking, but definitely useful for infiltration. The ship made a cameo in a Star Trek Armada II mission. The Romulans later steal the technology and use it to attack the joint Federation-Klingon Unity station. After the Klingons grumble about not being told about the tech, the Federation bans it too.
One episode of Farscape featured a planet where most advanced technology was rendered useless by a power-draining machine, set up to keep the colonists beholden to their original masters. Much awkwardness results when it turns out that Rygel is a descendant of said masters.
Reality TV shows often will ban certain items (such as cell phones or computers) during the course of the show. Ostensibly it'll be to encourage teamwork and communication, as well as to make sure the contestants aren't distracted by the outside world, but in reality it's most likely to increase the drama.
- Survivor is a particularly extreme case, as the contestants are placed on a "deserted" island and must live without basic amenities.
- There's also a whole subgenre of reality shows dedicated to historical re-enacments; shows like these would obviously prohibit contestants from using technology that wasn't available to the time they're re-enacting.
Firefly all but outright states that this is happening in the Rim and Border worlds, with the Alliance deliberately keeping the formerly independent worlds at a lower tech level than the Core worlds to keep them subjugated after the Unification War ended. At least one Border world baron has access to enough money and technology that he could easily build a modern city but keeps everyone at pre-1900's tech because he wants to be the one with the hovercar and handheld lasers while everyone else rides around on horses.

Tabletop Games

In the Dungeons & Dragons expanded second edition rules book High Level Campaigns, it's mentioned that, when designing a plane of existance, it can be given a tech level. Technology above that level will not function, unless the tech level is at least five levels higher than the magic level.
Gamescience's Superhero 2044. In the standard campaign setting, the World Council of Peace's Science Police is devoted to ensuring world peace. They control the creation of new inventions and confiscate any devices that could lead to another World War, including weapons of mass destruction.

Western Animation

In the pilot of Ben 10 Alien Force, Ben and a Plumber come across Kevin selling "level 5" alien technology, at which point the Plumber points out Earth's only cleared for "level 2" technology.
On the Men in Black animated series, the agents mention that some alien technology should not be discovered by humans until X years later.

Real Life

The Morgenthau Plan would have forbidden Germany from any sort of heavy industry, reducing it to an agricultural nation. As Herbert Hoover pointed out, for the idea to have any chance of working, Allied forces would have to exterminate 25 million people or move them out of Germany.
To some extent, the world community's treatment of Iran and its nuclear program. Through sanctions and other actions, the world community is "preventing" Iran from achieving nuclear technology, due to the fears of Iran developing a nuclear bomb.
The Amish purposefully police themselves, in terms of technology, as they believe in a simpler kind of lifestyle and think that modern technology will tempt and lead people away from religion/God.