The idea is to connect known spammers to a tarpit like spamd instead of the real SMTP server. The tarpit does nothing else than slowing down the spammer and not deliver his mails.
A pretty sophisticated setup is described by Benzedrine. His setup uses SpamAssassin along with procmail to generate a list of spammers. This list will then be used by PF to redirect spammers to spamd. There are some more goodies built in. Just check it out!
