Fibrestream (ex skifi) debits Bank accounts without debit authorisation

Sometimes in November 2019 Fibrestream took over my previous provider Skyfi and asked for a debit order to be issued to the new entity. I refused as there were accounting problems with Skify before and did pay my monthly account on time by EFT. Early 2020 I cancelled the account due to a move and settled all bills including the last month cancellation. Nevertheless Fibrestream debited an amount of R999 from my account on 1 April though they did not have any debit authorisation by using information from Skifi accounts and has since then refused to refund the amount.