Most spam messages contain links and most spambots don't register on the sites they want to spam. BlockAnonymousLinks blocks comments from anonymous users that contain links.
If an anonymous user tries to post a comment which contains a link, he/she will get a message explaining why the comment has been blocked. As an admin, you can change that message using the standard translation features of Drupal.
If you don't mind sending user content to an external service for spam analysis:
- Mollom only presents a CAPTCHA if the user input (e.g. a comment) looks like spam. Also provides stand-alone CAPTCHA service with intelligent, reputation-based CAPTCHAs.
- AntiSpam lets you use Mollom alternatives: Akismet, TypePad or Defensio.
If you don't want your user content to be sent to an external service for spam analysis, use Honeypot.
Last resort - because it always requires extra effort from your website visitors while there are smarter ways to handle this - a traditional stand-alone CAPTCHA module.