Vodacom not refunding my money

Vodacom debited my account at the beginning of March for an amount of R1100 instead of the usual R350 that is debited monthly. When I called the call centre, they admit they've made a mistake and that refund will be issued into my account. I've called over 5 times and every time I get told that the money will be reversed into my account within 7 days, its been over a month and i still haven't received my money. To make things worse, they then debited my account again at the end of March, while I'm still owed money by Vodacom, they take even more of my money. I want this sorted immediately, I'm sick and tired of calling and trying to sort this out.