Unused item being charged for

I currently hold a contract with MTN. I chose to upgrade from a Blackberry To an IPhone close to two years back. I continued to be billed for the blackberry internet service after my change of contract, which I queried instore and same was removed from my billing invoice thereafter. I am unsure if I was even refunded for this? I happened to look at my recent billing invoice to find in my horror that MTN have proceeded to charge me an additional amount for the months of August, September, October & November for a Blackberry service that I make no use of! What has gone on MTN, is this a system error? I require a quick correction of this error and a refund/credit to my account for the inconvenience caused. This should not be happening to customers.