The AP HoneyPot WordPress Plugin, by v-media, janstepien, madeinthayaland and M66B and based on Jan Stępień’s http:BL, allows you to verify IP addresses of clients connecting to your blog against the Project Honey Pot database.
Thanks to http:BL API you can quickly check whether your visitor is an email harvester, a comment spammer or any other malicious creature.
Communication with verification server is done via DNS request mechanism, which makes the query and response even quicker. Now, thanks to AP HoneyPot WordPress Plugin any potentially harmful clients are denied from accessing your blog and therefore abusing it.
Download: http://wordpress.org/extend/plugins/ap-honeypot/