I keep thinking that I finally have all the comments closed on my old posts, but the spammer's bots (same ones) keep finding open threads from 2002. So I keep deleting and closing the comments. Eventually we will run out of old, open comment threads. Maybe.
What seems to work for others is to force anyone writing a comment to preview the comment first. The spam bots can't do that yet. If you want to see it in action take a look at Jennifer's site: http://www.fortysomething.ca/ When I get a chunk of time to redesign my comment box I'm going to do the comment preview thingy.