Unauthorized debit order

I had a contract with MTN for 36mnths but till today they still debit me even though I've finished paying and they told me my contract will be canceled after the finished payment and my debt was paid up. But their debit order still goes through and I was shocked to receive a debit order from them today since I was using a debt collectors to pay and they told me my debt was paid up...please can I get help to cancel this contract and get my refund back this people can't be doing this to me