Bad Customer Service and refusing to refund me

Thrifty King Shaka Airport branch is refusing to refund me after they billed me incorrectly for fuel. I hired the car in April and returned it with a full tank, refilled at Sasol Filling station by the airport. It's been a back-and-forth with them since May. I have decided to report them to the South African Tourism and Consumer Goods and Services Ombud. Bad Service and I will never use them again