Unauthorized DEBIT ORDER MANDATE PUT THROUGH *****

I have an account with Telkom which combines of three different amounts. On Tuesday I receive a call from a sales agent who said that my contract on one of the accounts is expired and need to extend it. I explained to her that the said one is on month to month which is not a problem for me. She then said that taking the 24 month contract will be better. I told her I was not interested. On Wednesday morning I received a email from my bank informing me that a DEBIT ORDER MANDATE WAS SENT TO THEM FROM TELKOM. How the hell. What the F, who authorized that because I did not. I WANT THEM TO CANCEL THAT CONTRACT ASAP. AND LEAVE IT AS IF WAS ON MONTH TO MONTH