MTN won't cancel my contract

I called MTN to ask to migrate to prepaid on Monday. I was told it would take 24 hours. Today is Friday and my contract still hasn't been cancelled, although I have been contacted by the Retentions department attempting to retain my business. Every time I follow up via email on the cancellation, I get asked for my details, even if I have previously supp**** them to the same agent, but I get no response further than that. And when I call the call centre I get the run around, being told that I need to contact some other contact centre. This bad service is why I want to cancel my contract, if MTN would actually allow me to cancel my contract