Ask HN- What can I do to get rid of a serial spammer from my website?

I own a discussion forum and there is someone who repeatedly joins the forum to spam us with his drivel (articles he has written that paint him as an expert in something particular). We try banning email addresses and he gets new ones. We tried banning IP addresses and he gets new ones.<p>He is certainly in violation of the forum rules that do not allow for multiple attempts to join. Is there any recourse to get rid of someone like this once and for all? We spend way too much time on this spammer already.<p>Any thoughts? Can we sue him? Could we win an injunction or a restraining order? Could we recoup legal fees?