MTN insists I owe them though my contract was fully paid.

I had a contract with MTN years ago. I asked them to place a 30 days of cancelation because my contract was due to end and they advised that as soon as I make my last payment the contract will be canceled. They deducted their last installment and I thought everything is fine. The following month they kept deducting and I was reversing the payment because the contract was fully paid but they kept attempting until they stopped.

I just tried checking my credit report 4 years later and it's showing that I owe MTN an amount of R627, how's that possible when I paid them up? This is affecting my credit report and it is really frustrating. I had a bad service whilst I had contract and even after paying them off??