A wireless microphone with a timer
I've written before about microphones and asking questions at conferences. Having watched another crazy person drone on and on with a long polemic and no question, this time on a wireless mic, I imagined a wireless microphone with a timer in it. The audio staff could start the timer, or the speaker could activate the microphone and start the timer. A few LED would show the time decreasing, and then music would rise up to end the question, like at the academy awards. (In a more extreme version, those who did not turn the mic back off would get a small electric shock which increased in voltage, making it harder and harder to hold the mic.)
However, you do want a way, if the question is really interesting, to let the person speak if the moderator wants them to. This would suggest the music should come from the sound board and be optional. The electroshocks, too.