Paying for unused SIM Card

I have taken out a contract with MTN at the basic amount as advertised. The contract that I signed was for top up 100. I later requested that my airtime be topped up with R500 on a monthly basis. Due to the terrible network and poor service I had received from MTN, I went to another cellphone network. I called to find out if I can have my standard contract back and the additional airtime I requested cancelled because I am not using the SIM card anymore. I was told it is not possible to cancel the additional airtime as I will have to get a settlement for it, with the additions on it. I am happy with settling the initial contract amount but not for the added features which i have not been using and will not be using in future until my contract expires in March next year. Since July i have been billed R500 but i don't even use the airtime as the SIM is now dormant. Before i went to MTN, i was on Cell C, and all the additional features were cancelled with ease at my instruction. I am struggling to understand why i cant now pay for the standard contract. The call centre agent were not helpful at all. I need clarity on this because this is day light *******.