Vodacom contract impossible to cancel

I'm trying to cancel a contract since November without any luck. I accidentally upgraded a data contract in November 2018 on a contract I canceled 2 years ago. To my surprise it was never canceled and I paid for data I never used for 24 months. In November I got a call from vodacom asking me if I would like to upgrade this contract to a much better option. I agreed to the contract and after receiving the modem realized my mistake. I thought it was data on the phone I am using and not on the "canceled" contract of 2 years ago. I phoned vodacom and told them that I want to cancel this contract and they arranged for a courier to collect the modem from me. The modem was collected on 13 December 2018 by RAM couriers with Waybill number COL ********** 6. After a couple of phone calls to vodacom cancelations and unsuccessful visits to vodacom shops they finally assured me that the contract will end by the end of February 2019. Yesterday I realized with a shock that the contract is still not canceled. After phoning cancelations today they told me that the contract can only be canceled in November 2020 but they have logged a call and someone will get back to me on this. This is unacceptable as this has been dragging on for 5 months now and I keep on paying for something that I don't use.