If you have access to all the website files you could just stick a .htaccess file in there and ban his IP from that (Assuming everything is in one folder and you cant access anything else without going to that first).
If he knows how to get around .htaccess then you wont be able to do it that way, (If he can get around .htaccess I dont think you will get rid of him
) There might be some other way that I cant think of right now, but not knowing much about your fourm I really cant tell you.
You know his IP right? Dont want you to end up banning something you dont want banned if you dont know his true IP.