Slight car sabotage
I actually read of this on b3ta, in a reply to the "Road Rage" QOTW I think:

Wait until the middle of the night/he's gone to bed, and then liberally sprinkle iron filings over his car. They'll rust to the surface. It'll only be slight superficial damage, but enough to irritate him.

Alternatively, invite any friends you have with small toddlers over in the evenings and feed them a ton of sugar. As soon as he starts singing, encourage the toddlers to make as much noise as possible.

Or: listen to what he's singing/playing, and then try to sing/play along, one semitone out.
(The Empress it seems you CAN polish a turd!, Wed 12 Nov 2008, 11:23, Reply)