Prevent comments containing html links from beeing validated
Why can't you go for 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. Essentially, Mollom acts as a
  proactive content moderator that is on the job 24 hours a day, 7 days
  a week.