Two things to be careful of: 1) The artist may have different nicks on different sites. It would look really bad to accuse someone of theft only to find out that it is their own work. 2) What if the claimant is the thief? Although it's not likely to happen, it wouldn't surprise me if someone started claiming art that wasn't their own. (Either they're seriously desparate for attention or they have a grudge against the person whose artwork they're claiming.) Sure, they'd likely get caught relatively quickly, but the damage would be done. I haven't a problem with prosecuting thieves, but we need to make sure who's guilty before we lock and load.