Unauthorised debiting of my account

let me keep it short...... I app**** for a Data sim card online 5 months ago with MTN for R540 only to find out o the same day there was a promotion for the same data sim card for R299, when I called MTN to cancel, they advised that I wait for the delivery of the Sim card and take it to one of the MTN Stores without activating the it (which i did). To my suprise on the 25th of November MTN debited my account for R820, i immediately call them and they informed me that the sim card is activate and all this time i dont even have the simcard as I have taken it back. I Spoke to one of their consultant and asked them to block the sim card since they are charging me for something that i am not using and reverse the transaction from the bank.........AGAIN in December they debited R1100.....MTN I NEED YOU TO CANCEL THIS IMMEDIATELY....