In the speakers there should be two wires within the same covering going into the back. Open it up, scrape back the outer covering of the wires (with the power obviously off) and tape them together, making sure they touch. This will cause a minor short circuit when it's on and the stereo will jump like a badly scratched CD is being played. He'll get so pissed off that all his CD's are skipping, although they have no sign of scratches, he'll eventually stop.