Impossible to cancel a contract!

I canceled my contract August 2020, I phoned in to cancel the contract. I was told there is an outstanding balance on the contract in the area of R420. I made arrangements to make the payment online by giving my credit card details over the phone as I was told this is the easiest and quickest method. I confirmed the payment and was told the balance is now R0 and I should not get another phone call regarding the contract. I then received a snotty SMS from Virgin Active last month stating that my gym fees had not been paid so my contract was canceled. Today I got a phone call stating that there is R500 outstanding on the CANCELED contract? When is this going to end? I refuse to pay a single cent further until I get a written guarantee from Virgin Active stating when they are going to stop generating outstanding fees on a canceled contract. This is pure money making and is utterly ridiculous!