DNSBL - <a href="http://en.wikipedia.org/wiki/DNSBL">Domain Name Service Block Lists</a>.
This functionality allows to block by IP address if it exists in one of available blocklists.
Originally it is designed to identify email spammers, but it is suitable for websites too.<br /><br />
Use many services which provides lists of spammer IPs for blocking.
You can refer to the following sites for more such services: <br />
<a href="http://en.wikipedia.org/wiki/Comparison_of_DNS_blacklists">http://en.wikipedia.org/wiki/Comparison_of_DNS_blacklists</a><br />
<a href="http://www.dnsbl.info/">http://www.dnsbl.info/</a><br />
<a href="http://stats.dnsbl.com/">http://stats.dnsbl.com/</a><br />
<a href="http://www.moensted.dk/spam/">http://www.moensted.dk/spam/</a>