Money taken without resolving issue

I receive 2GB data monthly for my broadband and what isnt used was carried over. I phoned MTN to ask for a special debit order as am leaving the country, the woman on the phone advised she couldnt help me as i had exceeded my data limit and the amount was now sitting at over R1000 and how did i know i wasnt going to do the same thing the following month. I was advised it takes 5 working days for them to look into it and get back to me and i explained they would have already taken the money from my account. I said what happens if you are wrong and they need to pay a credit back and thet said it would go into my account. I said i am leaving the country and closing my bank account and cant use MTN in the UK. She didnt care and said there is nothing they can do. I went on to MTN Active which shows i have 2.17GB data left, which shows .17 was carried over, if it wascarried over how could i have exceeded my limit. I even took a screen shot of the page. Noone at MTN can help me with this but they can take R1000 from my account wihtout even knowing if its correct.