Yer a private entity, not a public forum. It's not the same. You're also doing M.Sippy a favor by deleting comments purporting to come from him that you know are from a hijacker. No one is born here or forced to come here or prevented from leaving if they don't like what you do. Filtering the content of your personal site is not censorship.
Censorship is when the government limits a person's ability to speak.
You are not stopping this person from speaking. You are refusing to give him a forum to speak.