Once, the Drupal community had Mollom, and everything was good. It was a web service that would let you use an API to scan comments and other user-submitted content and it would let your site know whether it thought it was spam, or not, so it could safely publish the content. Or not. It was created by our very own Dries Buytaert and obviously had a Drupal module. It was the service of choice for Drupal sites struggling with comment spam. Unfortunately, Mollom no longer exists. But there is an alternative, from the WordPress world: Akismet.