Okay. Blocking perverts is a legitimate cause and being able to ban people effectively is probably easier than implementing feature extraction from video.
However, people can make throwaway facebook accounts just as they can make throwaway twitter/Google/OpenID accounts. So, if you want something more effective, you could either:
* make a video chat that is not free-for-all. Back in the day, ICQ with video chat turned into a circlejerk because anyone could contact anyone. On Skype, people who you don't know cannot contact you by default, so it doesn't have that problem, but feels more like a videophone than a chat.
* use more elaborate rules: use the sender IP to recognize duplicate sign-ins, with an option of contacting support if you're caught wrongly by the ruleset.
* Provide an outlet for the circlejerk people so they can be among themselves (i.e., like having a /b/ on 4chan so the other channels can be civilized most of the time). For example, have a "ban this user" function that bans that user for the person only, but have a default that contact requests from people who are banned by three different users are denied unless they're in your "friends" group. Have an option to change the threshold so that perverts can still play pervert in front of other perverts.
* provide some kind of "namespace" and appoint trusted people as moderators for parts of that namespace (e.g., have a channel HorsesLovers that has its group of moderators). Having moderators per channel allows (i) people to develop a sense of responsibility for "their" channel, but also (ii) to limit any kind of dictator-like behaviour.
* I was considering whether asking for some sort of micropayment (i.e., pay 2$ or have someone invite you) would solve the problem, but I guess there's enough perverts with deep pockets that it'd only be a partial solution.