Still charge a canceled contract

In December I have canceled two wifi modems Vodacom send me and was insured that they will refund me on the money they have taken on my acc. Instead they still billing me for 2 modems I don't have. I'm sure they are not only stole my money but a lot of other customers money as well that is not checking there statements. This is after I try to get statements from theme for longer thane 6 months. I have receive for the first time in 6 month statements and they bill me for that every month and the they charge me for non existing contracts. Vodacom have no serves but like to stole from customers