Can't cancel contract

My contract with Telkom expired on 04 Nov 2017, I gave notice 30 days prior to this on 5 Oct 2017. Since then I have called their call centre more than 20 times and mailed them on a each address i get given to email the cancellation to. They claim they tried to call me and there was no answer, however there were no missed calls or messages on my phone. I have been promised first by the call centre agent ********** and then a few days later by the call centre supervisor ********** that if i sent the cancellation form to them they would pass it on the the cancellations department and the cancellation would be processed within 24 hours. Well no surprise that didn't happen. So now we are 2 months since i submitted my cancellation and no response. Last night I received an invoice from them for another months subscription, so they are happy to keep taking my money. That is 2 extra months subscription they are now taking, which they are not entitled too as the contract is over.