Laws prohibiting junk faxes are already on the books:
The Telephone Consumer Protection Act of 1991 ("TCPA")
The TCPA (47 USC § 227), and its implementing FCC regulations ( mainly 47 CFR § 64.1200) prohibits the transmission via facsimile of any material advertising the commercial availability or quality of any product, service or property to any person without that person's prior express permission or request.
Under the TCPA, recipients of unsolicited fax advertisements can file suit in state court to collect the greater of $500 or actual damages for each violation, and/or obtain an injunction (a single junk fax can, and often does, contain multiple violations). If a court determines that the violations were willful or knowing, the damages can be tripled at the discretion of the court.