Gumshoe mainly a list of keywords which if posted auto bans you if you have < n posts and use > y keywords in your post. Don't want to go into too much detail but that's the most effective. Keywords are super specific, if a new spammer comes along I'll just add some more to the list.
Also blocking URLS (even in plaintext) in the message body + subject help.
And CAPTCHA on registration increases the cost to the spammers to sign up each time.