What happens when they register a fake account? Do they start sending out adverts / spam from that account onto the forums or what?
Going on what I've seen on other forums, you get spam floods. The bots will at the same time also be using the email accounts to broadcast as much spam as they can to as many valid email addresses as they can get hold of/harvest before the account is closed.
And it's an arms race - every time you create a server system that actively hunts and kills spam accounts you'll find that the botnet owners/makers create a countermeasure that makes it harder for them to be picked up.