CellC charging me months after contract has ended

CellC has been charging me for a contract which came to and end in May 2020 and which I confirmed to cancel.

Just before my contract came to an end I received a call from CellC asking if I'd like to update my contract. I declined and said that I'd like to cancel it.

A month later, I was charged the full amount. When I called to find out why, they said that it takes a month for the cancellation to take place. Every month until the end of August 2020 I have still been charged for the contract. Their reasoning is that they're still sending me data, therefore I need to pay for it. I did not authorise a continuation of my contract, I asked them to cancel it when my contract ended. Everytime I call it's the same story. No one cancels it like I've requested them to.

Who gives them permission to just take money from my account without my consent and how do I resolve this?