Instead of banning, be pre-emptive. New accounts should have Captcha on their first 50 posts. That is a time/annoyance hurdle that would deter pretty much any wannabe spammer, but is not onerous for a legit account that only posts 1-2 times a day.
There's open source Captcha solutions all over, so it's not hard to implement.
Hey your awake. I got sooo angry with these guys, I posted another MAY I HAVE YOUR ATTENTION thread.