GKD,
This is a problem for most forum owners, its a form of spamming, most forum give you a back link just for registering, filling in your profile including your website url.
The best think is to regularly blacklist email addresses this people uses and delete their address.
You can find a list of popular email address some of this bastards at
shame a spammer thread
You are welcomed to add list of email from offending people from your forum to the list.