I've thought about the EMP idea, but the incidental casualties (like, all the electronics in a two block area letting out the magic smoke) put me off the idea.
Instead, I visualize an old, high-quality, high-power horn speaker out of an old theater (something like
http://www.ebay.com/itm/JBL-2350-ho...dio_Speaker_Drivers_Horns&hash=item416e38044e ), driven by an amp capable of putting out 1KW at 2KHz to 10 KHz. Put it on an extendible pole (think a power antenna from a car, but with 3" diameter tubing), and have a 360 degree aiming capability.
When the system hears the thump-thump mobile approaching, the horn rises majestically out of a non-descript box in the yard to full height, snap-rotates to point at the offender and track them, then starts blaring some Supertramp falsetto like "Dreamer". Boy I'd love to see the look on their faces.
An alternative is a frequency-adaptive jammer. Use a high-gain directional antenna and a frequency-agile transmitter, run through RF frequencies from a couple hundred kHz to a couple gHz, and, by listening to the output of their sound system (you said you could hear it, didn't you?), automatically determine any frequencies that cause distortion, bleed-through, or feedback from their amplifier. Then, sweep your RF signal right around that point to find the loudest signal from their system as you couple into it.
Then increase the RF power by 20 dB.
Don't even get me started on motorcyclists with open-pipe exhausts.
/frank