Shlomoh, there has been a mod wriiten (only about 5 lines) about this, go to
http://www.adultjump.com/links/ In the Links Mods (somthing of that nature)it list url to it, it was written for 1.1 though, im not sure if it will work for 2.0 beta 3, you spicaify the domain names/keywords that can not be added there instally poped up with and error mess "The domain name whatever they enter that you dont want is BANNED from or site" You'll never get an email, or it wont show up in your database its just like them submitting a blank form,
Thanks
Joey