I am a business owner. I have recently found out from a couple of my clients that they have received a letter which was not signed. But within the letter it says that its from my ex employee. In the letter it states that I don't know what I'm doing, that I'm losing these clients money, pretty much slandering me and my company. None of this is true. How can I stop this? She is trying to ruin my business and reputation.

"She?" So you know who it is? Have your attorney draft a letter telling her that you will not tolerate libel and a lawsuit will ensue if she persists. Her defense would be either that she didn't write the anonymous letters (are you absolutely certain you know who did?) or that what she is writing is true, so be prepared to to prove that it's not. Also you need to prove that you have suffered monetary damage through her actions, such as loss of clients that wouldn't have been lost otherwise. Bottom line - consult your attorney.