Impossible to cancel paid-up Vodacom contracts

Vodacom gave me good service for a number of years until it came time to cancel a sim-only contract that I had maintained for multiple years. Unfortunately even with no device, and a fully paid contract, it's proving impossible to cancel.

I've now spent more than 6 months trying to cancel, phoning multiple times (at least 4 times). Each time I chat to an operator, they take down my details and promise to cancel the contract and that I will receive proof of cancellation within a few days and a refund for the debit orders that have continued to be drawn even after cancellation was promised. Most recently, the operators have been hanging up after keeping me on hold for 20 minutes while they "process the cancellation".

This is absolutely frustrating and I cannot recommend that anyone takes on a contract with Vodacom knowing that it will prove impossible to cancel.

If anyone can help me resolve this issue, that would be grand!