What can I do about an ex-employee who is calling clients to go to her new place of employment for grooming their dogs?

Do you have a contract with her that prohibits her from competing with you after she left your employ  (even if you do, these types of agreements are often considered too broad and onerous, and are not always enforced by the Courts)?  Is she using any of your proprietary information/trade secrets in her new employment (proprietary information can in some cases include customer lists, if the information contained therein is not readily available to the public, and you spent time and money compiling it, but probably would not include customer information your former employee merely remembered)?  If not, your former employee is free to compete with you.