slander

I am the assistant director of a private day school in TN. One of our parent's (who just received notice that her family is being disenrolled in 30 days)has accused me of saying something that I did not say. She would not tell me the source of her information. When I told her that her information was incorrect, she accused me of lying. Her tone of voice and language were very demeaning.
My question is this: How can I inform her (in writing) that her comments are not appreciated, that I will tolerate no more, that I refuse to converse with her (other than in writing) and that further action will be taken if she continues to slander me?
Thank you in advance for your help.

-Emily

Write her a letter saying just that! Send it certified/registered mail, return receipt required. Keep a copy and the return card.