How do I stop my MTN contract??????

My contract with MTN ended in September, and after I have given notice that I do not want the contract to continue, MTN keeps on billing me for the contract. I cannot get through to their call centre to sort this out, all of my mails to them are being ignored. They have confirmed by email that they have received my cancellation and their retentions department even tried to sell me a new contract when they got my cancellation, so they did receive my notice of cancellation, yet they keep on billing me! I don't know where to go with this, surely this is not legal to keep on billing someone after the contract period has ended and notice were given of cancellation? I am not paying the account and they have listed me as a slow payer, and I don't have any way of resolving this as the call centre can't be reached. I don't know where to go from here! The sim card was blocked last year already after the phone was ****** and I just let the contract run out without using the sim, telephone number or phone, and now I can't even cancel the contract after sticking to my end of the bargain????!!!!!!