MTN SA

About six months ago, I opened a Home Internet Starter Monthly data account (***) and found that it was inadequate for my requirements. I then went back to the Mall @ Carnival branch, where the account was opened, and was advised to upgrade to Shesha 600(***). I handed in the Home Internet Starter Monthly modem and sim card, cancelled, and was provided with a Shesha 600 modem and sim. I was billed for both the "cancelled" Home Internet Starter Monthly(***) ever since, despite complaining more than ten times at Mall @ Carnival branch. The response I get from the branch there is nothing they can do except resend the cancellation which they did more than 5 times without a resolution. They are still double dipping as we speak. Please help , Account number BA***