Spammers generally use comment feature to post spam links on your Drupal powered websites. Here is how you can control spam comments to large extent.
Use Mollom Module
Mollom is a great Drupal module to control spams. Mollom handles incoming posts intelligently, in much the same way a human moderator decides what posts are acceptable.
Use CAPTCHA Module
A CAPTCHA is a challenge-response test most often placed within web forms to determine whether the user is human.
Make Preview comment necessary
Enable Preview comment necessary from Administrator >> Content types >> page / story >> Edit >> Comments settings.
You can further control spam, by allowing only registered users to post comment.