I am expected to pay for a cancelled service

I had a month to month contract which I cancelled (according to my knowledge) only for MTN to try and debit my account again. I was advised that my contract was not cancelled on their system and was that I should pay MTN ( I have never missed a payment). I am expected to pay for a service that I am not using because the MTN store in Norwood did not submit my cancellation request on time. This is ***** if you ask me