I dunno if you would call it a "forum" but me and a bunch of guys from 4chan used to troll the hell out of stickam.com. Basically we would get videos of creepy old guys on webcam, broadcast/stream them in girls rooms, then bust out typical pedo-lines like "mmmm your looking good tonight princess" or "do you have a myspace cupcake?" the reactions we got were absolutely priceless. Probably got banned 100 times or so lol.
http://oi54.tinypic.com/207vxw6.jpg