Kryptonite Ring

"Superman: I have many enemies who have tried to control me. And I live in fear that someday, they might succeed. If that should ever happen--if I should ever lose control--there would only be one sure way to stop me. Batman: Do you realize what you're asking? Superman: I do. I want the means to stop me in the hands of a man I can trust with my life."

In an optimistic setting, this device is from a character who is aware of their Kryptonite Factor and puts the means to stop them in the hands of another (usually, in case they're Brainwashed or go evil somehow). This usually implies a bond of trust since the receiver often has mixed feelings about the idea and the giver has to insist. This is also a show of good faith to a skeptic so he will allow the hero to operate in a legal grey area.

This trope is named after the ring Superman gave Batman if he ever got out of control.

Compare Betrayal Insurance where the "stopping" character came up with on their own; exactly how extensive this planning comes off can seriously affect the relationship depending on how betrayed the other feels.

Comic Books
"Hawkgirl: Do you always keep that in your belt? Batman: Call it...insurance. (grapples away) Hawkgirl: And they call me scary."
 * Professor Xavier also had a set of plans on how to stop the X-Men, Xavier himself being the subject of the first entry; however, these have rarely been mentioned since they were first introduced.
 * In Frank Miller's dark, near-future miniseries The Dark Knight Returns, Batman actually uses his Kryptonite Ring (the cynical version) has Green Arrow shoot Superman with a Kryptonite arrow to beat the crap out of Superman. This inspired the writers to have the Canon Superman give Batman a Kryptonite Ring (the idealistic version).
 * This specific dynamic, Batman holding Kryptonite, appears again and again, especially in the DCAU. During the height of the Cadmus story in Justice League Unlimited, Batman asks Superman what could stop the latter from taking over the world, if he so desired. Supes quips, "There's always that Kryptonite you carry around." Which makes Batman snap at him for being so flippant about so serious a topic.
 * Don't forget the piece he uses against A.M.A.Z.O. when it copies Superman's powers (and weaknesses.)

"Kal-L: "But the Kryptonite here isn't from my Krypton. It doesn't hurt me... physically, at least. But that ring [...] represents the paranoia and mistrust that will destroy your world if you let it."
 * The Batman has an incredibly cynical version. When Batman got a piece of kryptonite off of Metallo, not only did he keep it, he lied about giving it back (he gave Supes a fake one and kept the real one). He did it to "get even" with Superman for finding out his identity. However, by the end of the episode, it's the idealistic version, as they have settled their differences, and when Batman offers to give Supes the real kryptonite, Superman says to keep it. It still goes to show you that Batman really doesn't like anyone being nearly as Crazy Prepared as him.
 * Why would Batman suggest that he give Superman a piece of his greatest weakness? Pretty sneaky, Batman!
 * This is also inverted by the end of The Batman : Batman gives each member weapons that simulate each Justice League member's weaknesses because aliens have invaded and copied their powers. They call him out on being so paranoid, until Superman steps forwards and cites the time he was mind controlled in a previous episode.
 * The Batman/Superman comic combines the idealistic and cynical versions: after a long-term mission to destroy all the kryptonite on Earth, Superman saves one piece, which he gives to Batman for emergencies. Batman takes it to a lead-lined room in the deepest area of the Batcave... which is already filled with multiple kinds of kryptonite!
 * The trope is referenced, as always in combination with Crazy Prepared, when in the first episode of Batman the Brave And The Bold, Jaime Reyes asks his friend who he thinks would win: Batman (no kryptonite) or mind-controlled Superman. He then informs his friend, who answered "Superman", that it was a trick question: "Batman always has kryptonite".
 * To a lesser extent, the Shazam! Captain Marvel is employed occasionally as the one superhero tough enough to take on a controlled Superman if necessary. Or rather, he's tough enough to stall Supes in a fistfight long enough to call his magical lightning enough times to knock out his Kryptonian foe.
 * From the Legion of Superheroes example above: General Zod's son, Drax, stole Brainiac 5's kryptonite and tries to use it on Supes, but Superman kicked him through a conveniently placed Phantom Zone portal, before he could even open the lead container.
 * Pre Crisis, Superman's heat vision could melt kryptonite, rendering it useless somehow. This usually worked only on small pieces, though; in one instance a large meteor of the stuff fell next to him and incapacitated him before he could melt it.
 * Referenced in Kingdom Come, but it's noted that Superman has grown more and more powerful as he's aged and absorbed more solar energy, and Kryptonite doesn't really work on him anymore.
 * Infinite Crisis subverts this when Batman tries to use his Kryptonite ring on Superman of Earth-Two, which has no effect, as explained in explication.


 * In the Backstory of Ex Machina, The Great Machine (now the mayor of New York City) gave his two Secret Keepers devices to cancel-out his ability to talk to machines. As was his habit, he explicitly referenced the Superman mythos as he did so. As we later learn.
 * In Wolverine: Origins Wolverine gives Cyclops his magical sword, which is capable of slicing through his adamantium skeleton and negating his healing abilities, in case he gets captured and brainwashed.
 * Given his backstory (and more than one other story arc), that might be less "in case he gets brainwashed" and more "when he gets brainwashed. Again."
 * Of course his son now has it. Though at least Wolverine was able to use to kill Sabertooth first (not sure if it actually stuck).
 * There's no actual power-cancelling device, but Avatar: The Last Airbender - The Promise begins with Zuko asking Aang to kill him if he ever starts following in his father's footsteps. This comes directly after the series finale, which Aang spent trying to find a way around executing Zuko's dad (who is pretty much the most despicable man alive). And succeeded. He's not exactly thrilled, but Zuko insists he promise.

Live Action TV

 * In Star Trek: The Next Generation, there are various methods created to deal with Data to control him in case of possession or other matters. There is an off switch located on his spine that only a few know about, and some Applied Phlebotinum to disable him for extended periods.
 * In The Sarah Connor Chronicles, Cameron installs a bomb in her head and gives John the detonator, in case she ever becomes a threat to him.
 * In Smallville, Ollie Queen "acquires" Lex's Kryptonite Ring, confirming his role as Smallville's Batman. (Well, except for the Thou Shalt Not Kill thing.)

Film

 * In The Dark Knight, Batman gives Lucius Fox sole control over

Web Original

 * The Global Guardians keep a locked file in their comptuer database called the Code Red Omega Scenario. It is a tactical plan to take down each and every member of the Guardians, if it ever became necessary to take them all down.

Western Animation

 * Each of the Silverhawks had an emergency off-switch to be used if any of them went berserk for unforseen reasons.