You would have to sue the person for trade libel and intentional interference with contract and prospective economic expectations.
Trade libel occurs where a person publishes false and reputation injuring statements of fact about a person's business activities, and causes damages. This is a species of defamation of character. To prevail, the plaintiff must show that the statements are not merely the opinion of the defendant. The defendant must state false facts in order to become liable for libel (no pun intended). Example:
1. I think John Doe is mentally ill. (opinion: not legally actionable, no matter how offensive)
2. John Doe is a NAZI. (actionable libel: assuming that John has never been a member of the NAZI party)
Intentional interference with contract and prospective economic expectations occurs where the defendant interferes with an existing or prospective contractual expectation, for a purpose other than fair competition. The general rule is that the defendant must have malicious intent (i.e., the intent to injure the plaintiff as a punitive measure, rather than to try to successfully outperform the plaintiff in the marketplace).
Sometimes, assuming that the facts are favorable, a letter from an attorney threatening legal action, unless the defendant ceases and desists can scare the defendant into silence. If not, then you would have to sue, at which point, you're likely to get a settlement, because the defendant most likely cannot afford to defend a lawsuit of this magnitude. Especially an unemployed defendant.
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