Unsolicited fax spam is illegal under the Telephone Consumer Protection Act of 1991 (TCPA), as amended by the Junk Fax Prevention Act of 1995. Unfortunately, that hasn't saved me from getting in some fax spammers' databases -- even though my phone number is in the National Do Not Call Registry!
Luckily (?), I have a fax machine on my line, so at least when the fax spammers call at 2:00 a.m., the fax machine answers and receives the fax so that they don't keep redialing every five minutes all damn night.
The Unlawful Call Center helps you deal with unsolicited faxes by informing the fax spammers that you do not want to be disturbed (and that what they are doing is illegal). However, if the fax spammers don't stop, your only recourse is legal action (through a private lawsuit or complaint to the FCC or your state attorney general).
For the Unlawful Call Center to help, you must secure a "call trace" on the unsolicited fax spammer. If your local phone provider is Verizon, let the phone ring at least twice, then after you pick up the call and realize that it's an unsolicited fax call you can hang up (or let your fax machine answer so the fax spammer doesn't redial) and dial *57. This activates the call trace feature, which logs the true originating phone number in Verizon's phone records (you are not provided that number). After a successful trace, call the Unlawful Call Center at 800-518-5507, and tell them that you activated a call trace on an unsolicited fax spammer. The Unlawful Call Center will obtain the originating phone number and call and/or send a fax notice to the originating phone number informing the sender that what they are doing is against the law and requesting that the sender stop sending you faxes.
Again, the telephone number for the Unlawful Call Center is: 800-518-5507.