How to prevent from getting spammed in a contact form
That is why Mollom exists: http://drupal.org/project/mollom


  Mollom may block a post outright if it is from a known spammer. If
  Mollom is unsure how to classify a post, it may require the completion
  of a CAPTCHA to accept the post. Posts that do not match a "spammy"
  text pattern and do not originate from known spammers are accepted
  without the need to complete a CAPTCHA.
  
  A stand-alone CAPTCHA solution, which neither considers user behavior
  nor point of origin, can never achieve this level of informed
  protection, and generally requires users to solve a CAPTCHA on every
  post. Using Mollom's text analysis, users must only solve CAPTCHAs
  when Mollom is unsure about a post.