Unauthorised debit orders

i have 2 loans with FNB which were in arrears and i received a call from a lady by the name of Thulile Mabule regarding offering me a 20% discount to settle the loans so i asked her to e-mail me the discounted amounts so i can go through them as i have other debts with other financial institutions.A day later an amount of R15180.14 was debited from account without me givoing any go ahead when i phoned the bank they sending me from pillar to post,and now they have debited an amount of R2576.93 again without my knowledge,what i want the bank to do is to reverse the debits so I can tell them myself to debit the money not just to do as they pleased becaused they have access to my account.