yes, this has certainly been discussed before. blacklists are used in many ‘hacks’… I use it in referrals, but then I also use the referral blacklist to check against the IP and url (and the comment) to find illegal IPs or URLs being used anywhere. However, I simply have the comment system then mark it as unapproved, so I can see the spammers. You could just as easily not do the INSERT into the table if anything ‘matches’ the blacklist.
search for blacklist, you should find a number of people (including me) discussing the topic(s) further.