Vodacom incompetence

Good day, I have a complaint with regards to Vodacom. I have a contract with Vodacom, so what had happened is that they have overcharged me on my debit order on an update that I did not give an approval to. So I have two phones on contract with them, so my second number they had put it on an open line which means once my top up data and minutes are finished, it automatically turns into prepaid and whatever I have used for that month, they will bill me on it, so I had asked them to change it for me and remove the open line option. Only to find out later the follow month when they have overcharged me that they had done a double top up on both of my numbers with no approval from me for them to do those changes, as I had only called for my one number for the open line to be removed, they then put a double top up on both my numbers in which I never called about both my numbers, but only one. When I called to complain to them they only changed back my contract to its original package and said they won't be able to pay back or even transfer my refund to the next month, which is very concerning as I didn't approve to any of the changes that they had made on my contract package. Please can you kindly assist me.