Wow George, it's funny you should ask this, because I help run a large forum and have had an individual who has been doing this exact same thing! In fact, I found this forum by searching the domain he originally registered under, etheria.net. What a coincidence, huh? That's your site!
To answer your question, yes, you can actually bring legal action against someone who repeatedly posts disgusting "scat" I AM A SPAMMER on your website. I know this because the forums I help run are owned by a large corporation with a skilled legal team, who has been looking into this matter since it started. Luckily, the person who has been repeatedly trolling my site actually sent us an email, again from etheria.net, admitting that he was the person doing this and would continue to do it unless we reinstated his account.
Good luck with your spammer, friend! I'm happy to know that we'll be dealing with ours in a swift fashion in the coming days!