There is an entire area of law that addresses junk faxes. This law as a general matter sets many limitation in the sending of junk faxes. 47 U.S.C.A. 227 et seq.
The basis theory on damages is that junk faxes cost people money in the form of wasted time, paper and toner. This is why there are statutory damages of $500 per violating junk fax.
The cases can be pursued in State or Federal Court and provide an award of counsel fees. If the Court finds that the violation was intentional the Court has the option to triple damages as punitive damages.
There are several sites dedicated to junk faxes. Web site.