Discovery Card Complaint

I AM LIVID with Discovery card. I phoned them a few weeks ago to change my banking details as I moved to another bank. Please note all I wanted to do was to change my banking details. Which should require me to just send a confirmation letter with my banking details as proof that I have left the bank! But I was sent unnecessary forms. An option where I ended clicking that they must debit the full balance amount was selected as I was under the impression that the option meant Discovery debits me with the normal instalment amount. I called Discover to reverse the amount today and they tell me they cannot do that as I selected the option and this can only change for the next debit order. I have debit orders and other debit orders to pay! How do you debit an unncessary amount out of my account and not be able to reverse. To let people into your account there must be a level of trust you have with them. That trust no longer exists between Discovery and myself. I DO NOT WANT ANYMORE FORMS FROM DISCOVERY!