Struggling to get my money back from Vodacom after I've send them many emails to refund me for a tablet I've send back to them

Hi, I'm soooo dissapointed at Vodacom for sending me a tablet, after a sales lady talked to me, and told me that the tablet was free, just to find out that it was not the truth!!! I asked her if I dont want the tablet if I could send it back and she said yes! Ram couriers delivered it the Wednesday(3 May 2017) the Thursday(4 May 2017) I went to our local Vodacom Store to find out if the tablet they've send me would not cost me anything more than my recent contract at Vodacom. The lady told me that the tablet is not free at cost, they are going to charge me more than what my contract is every month. (My contract is R288 every month) When she told me that I immediately phone the Ram courier guy and told him to come and pick up the tablet because I dont want it. The next day (Friday5 May 2017) the came and pick it up from my residential address. The end of that month I received a bill for the ammount of R685. They've put the amount of the tablet also on my bill after I've told them I dont want it and after I went to the lady at the Store. She also said she would write an email and tell them I dont want the tablet and that they should credit me with the money that they took. Eversince that month, I've send Vodacom approximately 3 or 4 emails and asked them to refund me for the extra money they took for the tablet I dont even have anymore, but they did NOTHING about that!!!! Everytime they apologize and give me a little bit of data but they still took my money for the tablet every month!!! You know,I'm a pensioner and it's a lot of money for me to pay for something I dont even have. They started to increase my contract bill from the end of May 2017 up to date. So I want to ask you to act on my behalf PLEASE!!! I'm so dissapointed and I want to stop this immediately! I have all the proof of sending emails to Vodacom and asked them to credit me. Kind regards Johan Meintjes