From an answer I gave to another question:

There are comment filtering services out there that can analyse comments in a manner similar to mail spam filters (all links to the client API page, organised from simplest API to most complex):

1. Steve Kemp (again) has an xml-rpc-based comment filter: it's how Debian filters comments, and the code is free software, meaning you can run your own comment filtering server if you like;
2. There's Akismet, which is from the WordPress universe;
3. There's Mollom, which has an impressive list of users. It's closed source; it might say "not sure" about comments, intended to suggest offering a captcha to check the user.

For myself, I'm happy with offline by-hand filtering, but I suggested Kemp's service to someone who had an underwhelming experience with Mollom, and I'd like to pass on more reports from anyone who has tried these or other services.