How to Stop Rampant Junk Fax?

Fascist Christ asks: "The company I work for, like many other companies, uses a fax machine. We get an annoying ammount of advertisements by fax and are looking to find an effective way to stop it. We have tried requesting to be removed from the lists, but we get too many to do this every time. One company told me that they get there info from infoUSA. Does anyone have any experience with this or other companies and know how to stop this activity without getting a lawyer involved?" How difficult would it be to set up a fax system that would use caller-ID blacklists before answering the line? At the very least you can block identifiable spammers by something that's harder to change than their email address: their phone number.