Someone should invent something along the lines of those remotes that can turn off any televison, so when the idiots pull up next to you you could just shut em down.
Did something like this as a class project/senior thesis in a design engineering class I took. We created an airgun that would shoot an extremely large capacitor with a magnet on one end and a trailing ground wire on the other. The object was for the charged capacitor to contact the metal on the car while the trailing ground wire, which flew behind the capacitor until it attached, contacted the ground and completed the circuit. Theoretically, the capacitor should discharge into the chassis and hopefully blow out any electronics. We had limited success. But still, wouldn't it be fantastic to toast one of these guy's stereo's...
Although drowning out 50 Cent with 12 Gauge is quite an appealing solution, too.
Touch an unpainted surface of the car with a stun gun.