What Can You Do to Stop Junk Faxes?

olddoc asks: "I am having a growing problem with junk faxes. Unlike email, it costs me money when I get a fax so junk faxes really tick me off. A while ago, I gave my number to a removal number and now I am getting more junk faxes than ever." What options are there for dealing with this? If you've also had this problem, what did you do and how effective was it in stopping unwanted faxes?