Unauthorised transactions by MTN as a contract user.

I have a contract with MTN and it has been over a year since using their services. Lately I have been having issues with them and inconsistent monthly instalments, they offered to upgrade my monthly plan and I refused because the last time they had offered me a migration plan they had said that it would not affect my instalments and it did as a result my instalment increased from R619 to R660. I called them again to cancel the migration plan and they said that they had cancelled it. But continued to deduct the R660 with out my permission. To my surprise they called me again this month on Thursday on the 27 of June and offered me another migration plan and I refused and stated to them that I am not interested because of their dishonesty when it comes to my monthly payments. After telling them that I am not interested they still went ahead to send me a statement stating that I owe R792.22 and to my knowledge I don’t know how they even got to that amount. I am definitely not happy with the way they are handling my funds seeing that they keep on debiting unauthorised transactions from my account. I can’t trust them with my finances because they are mishandling my money. And I would really appreciate it if they stick to the R619 that we had agreed upon.