Billed by Mtn after cancellation of contract

I have cancelled my data contract with MTN in April. On 19 April I made a cancelation payment to them and confirmed telephonically with them that the contract has now been cancelled. During Jul and Aug I suddenly started receiving invoices for the contract again. I have spoken to a customer care agent twice, and both times they confirmed it was a system error and it will be escalated. Money has now gone of my bank accout for this cancelled contract and I am no closer to finding a workable solution. My rights under the consumer protection act is now being infriged.