After you sign up and request an API Key,  head on over to PlanetOzh and pick up the PHP script he made to query the Project Honeypot’s HTTP Blacklist server and then block any offending IP that is logged there.  Most of these spammers and e-mail harvesters were caught using PHPot’s  honeypot script which you hide in your code, which spam bots will follow and then basically send spam to PHPot’s servers, therefore being "registered" as spammers.  On my web directory, I’ve already caught over 40 spammers and harvesters in the act.  By using PlanetOzh’s PHP script in accordance with the honeypot script you can block the already registered spammers before they have the chance to attack.

If you’re using WordPress, get the HTTP:BL plugin.