Hi!

I don't know about building a dedicated device, nor am I sure I like the
aspect of damaging something that isn't mine, but...I know what you're
saying.

If you were close enough, and they happened to be listening to over-the-air
FM radio, you could try a low power FM transmitter. Some of them do have
surprisingly good range and can cover/distort an existing station when close
enough to the receiver. I've got one that seems to be able to go about 30
feet or so.

Then there is the direct (and bold) approach--walk over, reach in through
the window and turn the thing off. Or if you're feeling vindictive, pull a
small mallet out of your pocket and thoroughly bash in the head unit. But I
didn't say that. ;-)

William