Vodacom expired contract cancellation NIGHTMARE!!!!

My contract expired in April and I informed Vodacom that I do not want to upgrade but onto a pay as you go option instead. I had a double deal contract and cancelled only to get a bill 2months later again. I paid and cancelled again. I kept getting calls from agents offering an upgrade and continuously advised them that I have already cancelled and they would say there is no cancellation on the system. 6months later, I get a bill again but the phone is blocked and I cannot use the airtime on the device. I have a reference number for the refund I am supposed to get but then received a bill a few days ago. I don't know what more to do as Vodcaom is messing with my credit scores and history. I am LIVERED!!!! Vodacom please give my money and sort the account.