Though I never have a problem with the guy since I have no interest with Knight of Cerberus, I would like to give you some tips on how to stop him due to some experiences. In my opinion, the best way to stop him is by (presumingly) report him to the community central & disabling his Yahoo Mail account so he cant make anymore trouble. My sister who had a Deviantart account once had a problem about this when some "female user" keeps posting links to porn sites by replying on her artworks, calling them "nice". She wasn't the only victim as there where many others. Most of them already reported that user to Devianart's community central & disabled but came back with a new account name everytime which is then disabled. The Deviantart users, including my sister have reported her again to the community central, this time to give her a warning from creating sock puppet accounts. The said person currently stopped by now.