IANAL, but I doubt that threatening someone with actions that are legal is against the law. I can threaten to eat a Twinkie unless you give me one million dollars if I feel like it. Or a more relevant example is a patent troll blackmailing for licensing fees.
Threatening legal action is obviously not against the law. I would think it's not illegal to threaten legal action you have no intention of taking, either (although it's bad form).
However, should it actually get to a courtroom, it would not be good to have to address a judge and jury and explain that yes, you spent weeks lying to this person, threatening them with material harm to their business, etc, but that's the only thing you were dishonest about in the whole ordeal.
