Controlling spam with postfix
Steve has a pretty good tip on rejecting spammers with Postfix HELO controls.
In addition to that, I also use a combination of spamassassin and header checks to drop spam at the door:
In main.cf:
header_checks = regexp:/etc/postfix/header_checks
In /etc/postfix/header_checks:
/^X-Spam-Flag:.YES/ REJECT spam
What happens here? Spamassassin adds the X-SPAM-FLAG header and postfix rejects the message based on that.. This of course is only recommended if you're certain your spam rules are not providing false positives.