Nope, I doubt if there's any. Imposters are mostly just tagged by DT members, but in regard, I think it's handled case by case. How do we know for sure that he's trying to be an imposter? Unless if he sent you a tricky PM claiming that he is the original yahoo, this shouldn't be a big deal.

This guy (BitcoinArena) has a very similar username to mine, but I didn't look at him as an imposter. He simply just came to do business here and perhaps when even registering, he had no idea that I existed.