Planet Fitness is not Listening

Planet Fitness has been harassing me for a payment for an account I do not owe. This according to them dates back to 2011, so 11years ago. They insist that out of the blue, 11years later, I now owe them money. Despite me explaining over and over that I do not owe them anything they persist and keep calling for something I do not owe them. They keep threatening me with legal action although I do not owe them anything. There should surely be something about this under law. This company cannot continue to just randomly demand cash and take legal action for something I do not owe them.