They are tricky and hide their referrer links well - usually provide the link back in the user's profile and not the article. That way every post of their's links back to their website indirectly.
If there is a way to flag new users posting web links in their profiles for approval you can quickly blacklist their accounts. Usually these guys link back to SEO search optimization websites...how's that for irony...
You can be really draconian and start blacklisting posters by country IP but you will start losing customers in Africa, Asia and Eastern Block countries. Do you get a lot of TBN users in Ivory Coast? ;-)