From what you describe, it looks as though a potential business deal has gone wrong and the other party is publicly retaliating against you for some reason. If what is being posted online is untrue and damaging to your character or your business, that is indeed defamation. Her talking negatively about you and your business is libel. It sounds as though you could use a cease and desist letter drafted by an attorney instructing her to stop the behavior immediately or be subject to a lawsuit. This usually works wonders.
Answered on Nov 14th, 2013 at 2:27 PM