It sounds like they do not have the features to lock down a specific address - a domain is an entity that holds a series of specific addresses, each assigned to a user. Domains are not connected with the user, they are essentially companies or entities in their own (maybe connected with a telecon) so blocking at that level would affect thousands of other potential users/Lurkers.

Not a tecchie, so I am ready to be corrected as necessary.

Regards