My father in law was a bit of a genius with electronic stuff - he made a device that he plugged into a socket on the dividing wall between him and noisy neighbour's - it was a bit like a wheel with a handle on it, a sort of spark generating thing- when you spun it it produced really loud crackling through the speakers next door, music went off immediately while they tried to figure out what was wrong. They ended up buying replacement hi FI equipment, but alas the problem persisted.
Probably as dangerous as hell, but highly effective