The best response to the multi-identity is simply not to respond. He finds that frustrating. Obnoxious cheerleading posts are designed to get people to call him on his multi-identity-ness.

Don't play the game. Just ignore the multi-identities. You can do that by clicking on the name, which takes you to their personal page. Underneath the avatar is the "ignore poster" link. Or, you can just read the posts, smirk, and move on without response.