Discovery takes unauthorised debit orders from my account

Discovery is the worst ever. I notified them that I would be going abroad and that my cellphone number would not be working, and that they should please make contact via email about my savings that had to be paid out. One year later, and I find out that they had handed me over because my savings was used up and I had a last appointment at the doctor which made the savings account go into a minus. They did not send me one email to notify me about this. Then I made an arrangement to pay the amount I owed to them via debit order after I found out that there was already a default on my credit record lodged by them. The amount had been paid in full by 09 October 2017, and they even sent me a letter stating that the balance is 0. THE DEBIT ORDER WENT OFF ON 1 NOVEMBER AND 1 DECEMBER WITHOUT MY CONSENT AND EVEN THOUGH MY BALANCE WITH THEM IS ZERO, AND I EVEN HAVE A LETTER FROM THE STATING THAT MY ACCOUNT BALANCE IS ZERO. This is just terrible and no one can help me to sort it out. Do not choose Discovery, ever,