Tragic customer services to cancel contract

It is almost i possible to change from contact to prepaid. I regret to say that vodacom customer services failed very badly, as they simply cannot understand when I want to change to prepaid. It has been almost a year since they gave empty promises staying they would stop the contact, but they haven't done anything up until today! I say again, they were requested to cancel my contact, but they reuse to do as they promises.