Over billing and no refund

I am so sick and tired of Vodacom from expensive calls to over charged and billing. They nicely debited more than they should from my account but almost double and have not corrected it or acknowledge that they messed up. I am now short on my other debit thanks to their **** up... I wish I could afford to cancel my contact as this is not the first time that they have Royally ****ed me... Can anyone inform me what my next step should be? Thanks