With a honey pot.

Project Honey Pot [is] a trap for spammers. Spammers use crawlers to crawl through every page on a Website for valid e-mail addresses, and then add these addresses to their database.

Any Website operator can download the Honey Pot software and it will set up a dummy page that gives a fake, unique e-mail address to the crawlers. When spam comes in to that unique address, it’s a double gotcha; both the IP address of the crawler that harvested the fake e-mail address is known, and Honey Pot also scores the IP address of the sender of the spam.

As a result, Honey Pot has collected 2.5 million IP addresses of spam senders and 15,000 IP addresses of crawlers. Now comes the one-two punch. The company has released what it calls the http:BL, a blacklist of the 2.5 million compromised computers.

Most spam today is sent out by a compromised computer with a zombie, or bot (define) installed on the computer. The users of these computers almost always have no idea they are compromised, because they have no antivirus software installed to stop such infection in the first place.

Well, with the http:BL they will find out. The blacklist can be installed on any Apache-based Website, so when one of the 2.5 million IP address with a botnet running on them visits that site, the site can deny them access to the home page and inform the user of their infection.

Punch number two is for the 15,000 IP addresses of crawlers. Those are the people collecting and selling e-mail addresses. Harvesting is a slow process and botnets are expensive to rent by the hour, so the spammers do it themselves, on their own computers with a constant connection, since one is needed.

Source.

Head over to Project Honeypot to install a honey pot on your site, thus weakening the power of the spammer and keeping yourself, your site, and your visitors safe from spam!