The boom in facsimile machines has created a new marketing phenomenon: junk fax, the practice of sending unsolicited advertising through fax machines. Right now, the technique is used mainly by dealers in fax equipment and supplies. But, if it catches on, it could create a backlash. The problem: the advertiser uses the recipient's paper and keeps other transmissions from getting through. "We get a lot of complaints," says Jerry Brodsky of the American Facsimile Association. "People want to know how they can prevent it." One method: restrict distribution of your fax numbers.