I don’t know if if a link from a comment is worth something. I try to avoid deleting comments, even ones that I disagree with, but at the same time Google ranks those links with the same authority as links I make. I don’t have a stellar PageRank, but it’s high enough that someone wants to trick me into endorsing their site.
One other possible solution would be to rewrite all the urls in a post as http://www.google.com/url?sa=D&q=URLencodedLink which is what Blogger does to prevent Google from PageRanking links, to prevent this very problem.
Now I need to write that and tell figure out a way to advertise to comment spammers the fact that they will not benefit from comment spamming me.