MTN!!!!!

I have been harassed for the last number of years from credit firms, who now have been allocated 'my debt' with MTN. I had 2 mobile contracts with MTN some years ago. When those contracts 3-year period was up, i did not renew or take a new contract with them but had gone on to pay as you go service. I was, however, being continuously billed for the contract amount by MTN and had no joy going into various shops to try and address this. I now am being harassed everyday by the 'call center bandits'. Please can anyone direct me to any relevant person at MTN to sort this out. Rgrds.