Been billed for a contract that has been terminated.

My data contract with Telkom is fully satisfied and was on a month-to-month basis until I cancelled my contract with them using their online process whereby I gave them a month notice, they debited the month after notice was given and I accepted that. However, Telkom still debited me even after that, I reversed the debit order. I went to their shop and a billing dispute was logged and I was promised that they will resolve this and give me a call, to date no one from Telkom has called me. Telkom is still sending me invoices and billing me for a service they not even giving me. I am truly disappointed as the service and lack of customer care I am receiving, I even went on their online service to chat with a consultant and no one returned my called. I don't think I will ever go back to take a contract with them at this rate, however, can they get their billing department to stop billing for contracts that are cancelled.