Switching a contract to Prepaid

I have requested for a contract I had to be switched to prepaid from the beginning of February. Every time I phone the call centre I get the same response that \the line is still active and they will escalate the request again\". I have phoned 6 times and I even went into the Menlyn branch where they told me that I should give it time and phone the call centre again to confirm that this has been done. I get billed for this contract every month which is unacceptable and I now refuse to pay this account further. I will be cancelling all contracts I have with MTN as soon as possible as I feel this is not the way a client should be treated!!! How many times can one request a contract to be cancelled / switched to prepaid. I actually would love for the amount that I paid after the first request to be credited back to me as I did what I had to from my side.!! "