I came across this article on Blocking spammers with Postfix HELO controls after finding a log entry for a (rejected) spam that appeared to be getting sent from my own mail server. My own server never sends email (all that routes through my ISP), so this is all instant spam.
In the case of the mail that appeared on my system, a different spam filter caught it before it was even delivered, but I figured having one more hurdle for mail to pass wouldn't hurt, so this seemed a good one to try out.
I followed the steps in the article to set this up here, pay special attention to the 'Making it so' section about half way through. When this file is created or changed, you need to do a 'postmap helo_access' in the terminal for the changes to take place. If you don't, not only won't your changes take place, but postfix will log a warning during each server connection telling you that the source file is newer than it's database.