Block users from registering to your WordPress site with custom defined blacklists and whitelist rules.
Tornevall Networks DNS Blacklist support for WordPress
Botnet identifier using public DNSBL bases and a hardcoded whitelist/blacklist.
Free spam IP blocker according to public DNSBL bases.
Check if a visitor is listed on httpBL.abuse.ch, see http://dnsbl.abuse.ch/faq.php#about_httpbl for more information.