If she is going out of her way to speak to your references then I would absolutely send her a letter. I would say, as politely as possible, that your business is like any other, and is a first come first serve business. Inform her that whether or not a family is accepted into care is at your sole discretion, and that speaking negatively of you to your references does nothing for her cause. If the things are as bad as you imply then I would add a line such as, "If you do not cease to defame me and my business then I will have to treat this situation as harrassment and act accordingly within the legal system."

I'm sorry you are dealing with this.