movabletype.org: News: The new version of Movable Type includes spam fighting features.
As a stopgap before we release comment registration in Movable Type 3.0, we’ve released version 2.66 of Movable Type, which includes some protection against comment flooding. We’ve included a throttling measure so that comments from the same IP address can only be posted every N seconds, where N is configurable (documentation on the setting that controls N). We’ve also added a measure to automatically ban an IP address based on an abnormal number of comments from the same address in a short period of time.
And they include an idea I stole from…someone. I can’t remember who.
…we’ve changed the behavior of <$MTCommentAuthorLink$> to use redirects when linking to URLs given in comments. The goal of this is to defeat the PageRank boost given to spammers by posting in the comments on a weblog.