as days pass by

The latest comment spam technique here seems to be a script which looks at previous comments, grabs a paragraph from one of them, and posts *that* as a comment, with the URL being a spam URL. Anyone got any good ideas for how I can fix that? Since the earlier comments are legitimate, it’s quoting legitimate text and therefore no spamtrap in the world will catch it. I could hack WP to check previous comments for the text they’re posting, but that’s easily get-around-able by changing the comment spammer script to replace “o” with “0” and all the other tricks we’ve seen in spam emails (“C1Al15”, anyone?) Suggestions?