Hi AK,
You are asking for a software/hardware solution to a meatware problem.
The first suggestion I would make is allow the speaker to know the time.
There are several speaker timing and notification systems available
(yellow/red light on the podium type things). The problem comes with, what
do you do when the red light isn't enough? The speaker knows they are over
their time limit, but feels their talk is too important to cut off. Most
speakers are talking because they're excited about their topic, but the good
ones know how to communicate in the time allowed.
Steve's suggestion of an MC ending is a good one
I like the solution where you pull the Super-Soaker from under your
chair
A blast of 'accidental' feedback usually gets the point across, but also
upsets the audience.
You can try the Awards method, where you fade out their microphone and
fade up the exit music
It is kinda a shame that big white hooks went out of fashion
But really, it comes down to speaker selection. If a speaker is given a
set time frame, they should be able to come in close (barring special
circumstances like a fire drill in the middle of their talk - which could
also be a nifty solution as well <-- don't pull it, that was humor). Let
the speaker know in advance that the time frame is set and rigid, and that
they are expected to finish on-time. If they start asking what you mean by
on-time, you may want to look for another speaker.
Bill Dilworth