Sim card cancelation

I have a contract with Mtn which when I app**** for last year September when the contract was being processed they asked if I have an Mtn simcard already which I did at the time so I was told my contract will be done under dat sim card with number +*** now when I received the phone it had an additional sim card which I did use so I noticed that I was being billed on both sim cards so I emailed Mtn in January to have the sim card for +*** canceled and they agreed via telephone conversation on 6 January with a team leader from Blue label connect and I was told they managed to waive the cancellation fee but the following month i was charged again then I sent another email I was told they apologize and they asked me if I want to keep the number to prepaid I told them to cancel it completely which they did I speak the number is nolonger in use but I am still paying for it today 7April I called them again the said my cancelations request was rejected but how bcos the number is not even working anymore.