You could try the group thing again, I'd go much smaller and let these guys find it first to observe their habits.
I'd also be very sure before I accuse anyone of anything. If it's not them after all that's a serious foot-in-mouth.
If you figure out it is them for sure and they don't respond to polite suggestions, you could always try beating them senseless.
How bout a cache that appears on webcam, but with no mention of the webcam, so you can keep an eye on it? I screenshot of them ruining a site would go a long way. At least then after you posted that they would probably feel the pressure from the whole geo community and not just you.