Couple different ideas:
1. Use .htaccess as mentioned above to block his ip’s….
here are a couple useful threads on the topic:
http://www.webmasterworld.com/forum13/687.htm
http://www.webmasterworld.com/forum92/413.htm
2. Setup your blog software so that any comments have the link=nofollow specified in any posted URL’s.. so it doesn’t matter if someone spams you, as they won’t benefit in the search engines.