If you want to stop it, see if you can get some video footage including license plate numbers and faces, and put it up on youtube and facebook. Include pics of the nearby street signs. Advertise the crap out of it as if you're trying to get people to go see it (create anon "fake" facebook page, etc). Give it a week, and if it doesn't get broken up, send the video link to the local news station.
Of course do this very very sneaky and anonymous otherwise they'll beat the crap out of you.