Harassment from Nimble group for an account that I have never owed

I am being harassed for a zone fitness account that I do not even owe, I had a once off cash payment contract with zone fitness, when they check on their side I don't owe them anything but nimble group does not want to stop to harassing me with their phone calls they have blacklisted me, I have sent them all the proof that they requested but my issue has not been resolved till today