I would explain to her that your contracted services that were agreed upon by both parties have been achieved. Any other services above and beyond these agreed upon terms would be billed to her. I would then have a lawyer send her a letter advising her to stop her slanderous remarks to other people regarding you and your company. In that letter I would also have him to explain to her about possible litigation if she does not stop her verbal assault on you and the civil suit that would be brought against her for defamation of character and the big one, mental anguish as well as the pain and suffering you have endured from her hurtful remarks towards you and your company. If she doesn't back off then, I would go forward with suit. Hopefully she would be smart enough to shut that trap!!