Help me stop MTN from billing me on something I don't have

Hello Peter can you please help me, I once had a data contract with Mtn later on they offered me a laptop contract over the phone and I gave them a thumbs up after if was delivered it was not what we agreed on then I return it within the period given to make return if not happy. They received it but continued to debit for both contracts when making follow up I was told is not yet cantured that it was returned till I stopped the debit order with the bank. After a very long time of communicating via email they cancelled and cleared my name, I called after sometime to check if the contract was still on n they told me there's nothing showing on my name. Later on I gets email that they are billing me for a sim that they blocked years back and when I call they keep on transferring my call, on the branches they tell me that they can only accept payment the rest they don't have access for. Can you please help me