Incorrect billing

I was on an MTN contract and moved to vodacom. First they put me on the wrong contract and then I had to pay for it. I informed them immediately and they corrected it and put me on the Flexi 200. You get R200 per month airtime, no data or sms. This was beginning April and now they took an amount from my account 3 x my contract agreement amount. I contacted them and they said it was for data I used. How in the world do they get this?? I purchased my own data and did not have any on this contract. It is rediculous and not expected. I would rather go back to MTN. I want all my money back or cancellation.