What to do if I recently opened a martial arts school and people from another school are trying to spread negative information about me?

Question Details: They are telling people that I am claiming certain credentials, which I am not, and that I am a fraud. They have made the statement that they will "pay me a visit soon", which I think sounds like some type of threat. Is there anything that can be done? Also, I've already contacted them with a cease and desist letter, and they refuse. My issue is that they will potentially prevent me from having customers. They are also students and coaches from another martial arts school.

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