How to Block Spam Faxes (Score 1)

Any admins out there know of a good way to block spam faxes?? I can block by fax id on the fax server, but the majority of the fax ids of the spam faxes are blank. I also get legitimate faxes with a blank fax id, so I can't block them by fax id. I've contact the fax server vendor and they are no further help. Anyone know of any creative strategies for defending against this abuse?