Annoying call center agents

I have been with vodacom for close to 12 years, recently I only encountered problems and decided to switch to another service provider. My current contract was supposed to end the end of January, I phoned the call center to cancel my contract and I was told that the contract will cancel itself because I have reached the end, now according to my experience with vodacom your contract goes over to a month to month basis until you cancel it. Today 1st of February I received airtime again, once again I phoned the call center, I was on line for 45 minutes before someone decided to pick up my call, I told the lady I want to change my number to prepaid and cancel my current contract, she told me all sorts of stories and what else she can offer me, after telling her I am not interested I want to go over to prepaid she once again tried to pursue me into telling me how expensive it is, now I am getting really frustrated, as all I want to do is go over to prepaid, she informs me that I will still be billed end of february because I received airtime again, now if the first call center agent did her job this number would have been on prepaid before I received airtime again. After about 5 minutes I get told sorry sir our system is very slow, I can't go into your account to check your details but she could check what deals they can offer me. Cancelling your vodacom account is the hardest thing to do because everyone is making up stories as soon as they hear the word cancellation. I also get phoned over 20 times a day by the call center to offer me something and after blocking around 10 different numbers they try phoning from a private number, after losing my cool I told the call center lady where to get off and stop harassing me, if I wanted something from vodacom I will go in store. All I want from them is moving my number to prepaid so I can go on with my life