Cancelled Contract and they still bill me!

I had a contract with Autopage for a cellphone and a tablet. Autopage closed down and Vodacom too over my contract. In September I cancelled my account as it was the end of its term. My cellphone went over to Pay As you Go as well as the tablet. I suddenly received a bill from Vodacom that is overdue by more than R2000 on the tablet's nr for data usage. How can the tablet nr be overdue and not my cell phone as the 2 contracts were "together"? I am furious! I spoke with 2 people in the Vodacom Call Centre when I cancelled my contract in September. Vodacom emailed me to advise that they will credit my account for R600 ONLY. How can they credit me to my account WHEN I CANCELLED MY ACCOUNT? ITS PAY AS YOU GO! I want all my money back that was billed to me since September and I want it back in my ban account! I will never recommend Vodacom to anyone! You are just money hungry and have no control over your billings & accounts!