Bad credit record because of Discovery

I am highly disappointed because i now have a very bad credit record just because of one mistake done by Discovery. Membership number *** the agent/consultant Xolani Khoza around the 24th of December went ahead and opened a medical aid before telling me how much the premium would be. then when i discovered that already a medical aid number was allocated and a debit order was scheduled to go off, i asked him to immediately close that medical aid, and he assured me that it was closed same day and i wouldn't have any debits going off from my account. on 2 January 2016, a debit order went off on my account and because i had not made a provision to have that money deducted, it became a negative on my account. now discovery guys are not willing to assist me with a letter that states that it was a mistake on their side. noone is willing to assist me, last time i phoned Discovery in Johannesburg i spoke to a Sibu/Sivu guy and that was that i never heard from him, nor got the letter. no bank is willing to give me any loan at the moment, because of that failed debit order. can someone from Discovery assist!!