It's easy to shut someone up with a gun. It's harder to do it reversibly. Japanese researchers have developed a non-lethal gun that uses lasers (lasers!), microphones, and a directional speaker system to fire your words right back at you, jamming your brain and keeping you from speaking coherently.
This device is only a gun in that it has a trigger and you point it at people, but otherwise, it's little more than a fancy microphone and speaker system. To use it, line up the laser sight with the mouth on whatever obnoxious yelling jerk you like, and pull the trigger. A directional mic picks up on what said jerk is yelling, and plays it back at them, loudly, through the directional speaker with a delay of about 0.2 second.
And that's it. That's all it takes. As it turns out, it's impossible to talk (or yell) about anything with your own words played back at you with a sub-second delay. The designers of the "SpeechJammer," Kazutaka Kurihara and Koji Tsukada, say that the systems works better when someone is trying to recite a speech as opposed to when they're speaking spontaneously, and it (unfortunately) has no effect on yelling that isn't speech.
You can use your imagination as to what the SpeechJammer might be good for; the researchers themselves suggest (somewhat naively, we think) that it could "maintain silence in public libraries and facilitate discussion in group meetings." Um, great. We'd much rather that they come up with a pocket-sized version that you can take into movie theaters and restaurants, maybe with a built-in Taser just in case the SpeechJammer doesn't quite get your point across.