Frustrating times

My Makro account was handed over early 2017. March 2017 I received another letter of demand via email. In July 2017 I paid the full amount according to the letter received in March. I informed them on the 26 July 2017 that I have paid Makro and that my account should be closed. NO RESPONSE. Now 8 months down the line all of the sudden they keep harassing me due to unpaid amounts. When you talk to a faceless voice you are directed to a faceless supervisor who will call you back but never do. In the meantime you keep on being messaged and phoned. I need this to stop but dont know anymore what else I can do for them to get off my back.