cancelling of contract was never done!

I cancelled my contract in may from the call center. Before i put down i asked if it was cancelled and she said yes!<br> To my surprise come the next month i get my airtime and data etc. I went to an MTN store and they said it was never cancelled! I aske the guy at the store if he can cancel it, he said i can fill out the form but no guarantee, the best is through the call center, which isnt helpful, i reached out on twitter and nothing!