Passive weapon: Whenever a content or registration is sumbitted is first evaluated by specialized webservices like:ProjectHoneypot, StopForumSpam, FSpamlist, BotScout, Akismet, Mollom, Defensio, and some RBL and only after an "ham" classification are allowed.
Active weapon: These webservices databases are updated continuosly so maybe the spammer could be trapped some days later
This test will check the IP address against several DNS based blacklists. (Commonly called Realtime blacklist, DNSBL or RBL). If your ip has been blacklisted, some email/comment/registration you send may not be delivered. Blacklists are a common way of reducing spam.