A friend of mine works AV at shows that have rotating DJs and one of the things she has on her mixer board is "The Suck Button."

It causes a mic at the other end of the room to get cut into the DJ's live feed monitor with a semitone shift down and some reverb. This causes all sorts of inner-ear chaos and usually clears a DJ off the stage when they're over time within a few minutes at most -- usually under 30 seconds. One time they were trying to figure out why it wasn't working and discovered that the DJ had muted their monitor feed, which explained why they were not only peaking the meters but over time: They hadn't heard the FOUR warnings from the back of house that it was time to wrap up.

There was a coffee shop ages ago in SF that would every few hours play a cacophony (e.g. multiple songs at once). I assume it was to drive away people camping on their laptops to rotate tables. Understand but super annoying to people like me who had a timer to but food or drink no less than hourly to be a good citizen