Contract Cancellation

I contacted Cell C on 28 February 2017 (ONE YEAR AGO) to give 30 days notice for the cancellation of two contracts. Only one contract was cancelled. Despite numerous calls to 084-143 nearly a year later I am still being invoiced for the other contract. The contract I am struggling to get cancelled is a data contract, and the sim has not been in a device since February 2017. The standard response I get from the Cell C call centre agent is that they will request a copy of the voice recording and then follow up. There is never any follow up. I would like Cell C to cancel this contract and refund me the monthly subscription charged since March 2017.