Appalling and *********** service

I purchased a WiFi ****le on a 10GB 24 month contract in November 2016 at the Vodacom store in Canal Walk for R299. It was a promotional price. The original price was R499. This was supposed to be a top-up contract and not an open line. In March 2017 I received a bill of over R1000. I was then informed that the contract was an open line contract. I then had to go in to the Canal Walk store to present proof that this was supposed to be a top-up contract, which I did. At this point they reloaded my contract onto their system. A month later, May 2017, I received a bill of R499. I’ve been in to the Canal Walk 8 times, in addition to calling the store multiple times, over a 5 month period to ask for this to be corrected and it still has not been. I’m currently still being billed R499 which is completely ridiculous. I’m not sure what to do anymore.