Unauthorised debit order

I have cancelled my DSTV account in March this year. When I cancelled my DSTV there was a credit owed to me. As I have always paid via EFT, DSTV requested my banking details - which I provided. June 2018 - months after having cancelled my DSTV account, Multichoice decided to debit my bank account with a monthly subscription (which I don't have anymore). I cancelled the transaction via my bank and put a block on DSTV. Multichoice then proceeded to try and debit my account again that same month and then again for 2 months after that. For the past 5 months I have receive threating phone calls about blacklisting me for the monthly subscription that was debited and reversed. I have made numerous phone calls to the call centre, stood in the ques at the multichoice office in our town and no joy. It shows that my account has been cancelled and that all my payments were paid via EFT. But keep receiving theses calls and threats. Receive a call again yesterday again. This is now the last straw. How hard can it be to take me off your system. Not to mention the fact that multichoice authorised a debit order on my account without my knowledge or authorisation And without an active account - This is THEFT!!!!!!!!!