Unauthorised debit order

I cancelled my contract with home-connect on 24 February 2020 and received a message that my request had been received on 27 Feb. I paid for March (in advance) as I had to provide a month's notice. On 1 April I received a message that my cancellation request had been processed. I stopped the debit order. In September I received a message that I owe them R558 and that amount was debited from my account. I tried several times to call their accounts department for over a week (perhaps in excess of 10 times in total) but they did not answer the calls and did not respond to messages to return my call. Eventually someone from another department got them to contact me after I threatened to report them for *****. They assured my that it was a mistake and that they would return the money. I had however already stopped the debit order which cost me R40. I have since still received messages that I owe them R558.