You may not be able to do anything. If what is being said about you is an untrue factual statement, then you may have a cause of action, and be able to sue for either monetary compenation or for a court order barring such comments or statements in the future; that's because untrue negative factual statements are defamation.However, true factual statements, even if negative, are not defamation. More importantly, opinions, no matter how negative or hurtful, are not defamation, and you cannot take action over them. "Incompetence" is an opinion; this company can say they consider you "incompetent," just as they could say they consider you  were "creepy," "annoying," "anti-social," a "poor performer," "lazy," or anything like that, since all of those are opinions. What they can't do legally is lie about a non-opinon fact. Therefore, if they are saying you do not have experience, credentials, or training that you do have, or claiming you were excessively absent when you were not, stole from the company when you did not, etc., then you may have a cause of action. It must be a fact, not an opinion, to give you recourse.