Stan, you can harvest their IP's and start blocking those. There are lists on the Internet of IP's to block as known SPAM relays. You can also put in Captcha's and lock down the forum from posting URL's without verification of the URL. You can even block certain words from being URL's with the filters built into web forum software.

The number one thing you can do is block unregistered users from posting. The second thing is make the verification process either encrypted (using HTTPS) or human intensive (using automation and captchas) so that robots can't be used to post.