Chatroulette could allow users to flag the perverts, and also give users the ability to request "Please don't show me perverts". I don't see this as a difficult problem for them to solve if they wanted to.
Please share some other hard type problems from such sites if you feel like it. Then we can talk through/theorize about what might work in practice.
However, someone who uses chatroulette may have good reasons to flag every person they see as a pervert.
I'll work up a list of stuff, over the years I've spent many hours analyzing the abuse angle of webcam sites and while I haven't found a completely automated way to do it we have it worked out to the point where with relatively simple tools a single person can easily monitor a few thousand cams. Chatroulette is one-to-one, which has a bunch of unique problems in terms of the speed with which you can gather statistics on cams in order to determine the kind of content.
Ironically the best indicator for mis-labeled adult content is the number of viewers...