From MIT's Technology Review:
The idea is simple. Psychologists have known for some years that it is almost impossible to speak when your words are replayed to you with a delay of a fraction of a second.
Kurihara and Tsukada have simply built a handheld device consisting of a microphone and a speaker that does just that: it records a person's voice and replays it to them with a delay of about 0.2 seconds. The microphone and speaker are directional so the device can be aimed at a speaker from a distance, like a gun.
In tests, Kurihara and Tsukada say their speech jamming gun works well: “The system can disturb remote people's speech without any physical discomfort.”
Their tests also identify some curious phenomena. They say the gun is more effective when the delay varies in time and more effective against speech that involves reading aloud than against spontaneous monologue. Sadly, they report that it has no effect on meaningless sound sequences such as “aaaaarghhh”.