MTN, pay back the money!!!!

I went to MTN Kenilworth on 31/01/2018 to upgrade a contract after my daughter's contract expired. In fact she downgraded by keeping her mobile phone and changing to a month to month package from R614 p/m to R229 p/m. When I left the store the package was immediately downgraded. I received my latest billing statement March 2018 and again MTN will debit me for the full payment of the old contract. We have no new device MTN!!! I visited Kenilworth Centre twice, nothing happened. Extremely poor service at that branch where staff are on social media while dealing with you. I called the customer service centre on 22/03/2018 and was told they would log a service request. How ********* is MTN to debit my account for a device I don't have?? I threatened to return their debit if they do not refund me my money. I will definitely be changing to another service provider. If MTN cares to read this at all, my reference number is 1- ********** 7606.