While this is unlikely to be a broadly useful answer, here goes.
Some years ago I attended a focused conference/workshop held in a small town in Italy. The town had a lovely conference center with an auditorium and a nice sound system for the talks. Each day there were sessions with talks, as well as discussion time and activities. The first day's program was a disaster - nobody kept to their allotted time, the session ran late, lunch was late as a result, and so on. One of the more experienced hands said no problem, I'll take care of it tomorrow.
The next morning the first speaker ignored the stop signal and kept on talking. The audience then heard, slowly coming up in volume, a selection from the Three Tenors. The speaker tried talking louder. The music kept swelling. The speaker quit speaking, and the music slowly went away. The audience looked back, and there was the experienced hand at the sound board with his CD player jacked into it to provide the music.
The next speaker also ignored the stop signal and tried to keep talking. Once again, music from the Three Tenors started being added into the sound system. That speaker figured out what was going to happen, and stopped rather abruptly before the music got too loud, and the audience had a good laugh.
Nobody else ran over time through the rest of the week.
So, a sound board and an Ipod to provide lovely classical music is one approach to the issue. It works quite well.