Billing me after I canceled my sim months ago

I have canceled my rain sim card in November - so my last payment should have been 1 December. However, I was still billed for December (at the end of December). I called rain (at the start of January) to inform them of the mistake and they said that they would fix it and refund my money. I still have not received my money back for December. Additionally, I was billed again at the start of February for January. I submitted a ticket to ask them to fix the mistake. I got no response. Therefore, I submitted a second ticket a week later with all the reference numbers of my interactions with them. They still have not responded and they owe me almost R1000,00. I would not be surprised if they billed me again in March for February (even though the last time I used my sim was in November). The whole reason I got Rain instead of another ISP was because they advertised that I could cancel anytime. I do not know how many more times I should bring this to their attention.