What may work and has a little more comedic value and stealth than a
full blown gun is a pellet gun (air rifle). Make you own rubber
pellets (indian rubber is best) with a broken peice of cermaic inside.
Fire it at the side or rear window, the cermaic will puncture the
glass causing it to break into little cubes. The rubber will make sure
that it bounces away from the window (no evidence left on the seat)--
have you ever broken a window as a kid with a hard rubber ball, the
ball always came back as opposed to going through. Little sound from
the air rifle (they will never hear it).

In the end the idiots will think the sound system blew out the window.
"Man my system is powoerful, it blew out my window..." They may come
back but it will eventually get expensive for them....

Does not kill the music but it may solve the issue.... If nothing else
they will turn it down to save the windows...

Will not work on the windsheild BTW, different glass.