I came across this printout taped next to a fax machine today:
From the National Number Removal Assistance Association. They claim that if you fax your junk faxes to their fax number at 866-788-5202 that they will contact the people who are sending the faxes to have them remove you from their fax lists.
Not sure if it works, or if it is even worth attempting. As far as I am aware it is already illegal to send unsolicited faxes.