Unjustified bank charges, which led to Negative Balance of R650.00

I have a cheque account with no overdraft yet there is a negative balance of R642.88. I contacted FNB contact centre to understand why I have these charges on my account. The lady i spoke with confirmed that I don't have overdraft yet could not explain best to her ability to make me understand why am I owing FNB so much money. The only explanation was that I made a purchase at a Merchant which came 3days later as unpaid and that is shocking to me because everytime when I transact I always check my balances and after every transaction I get a notification stating that the transaction is successful and my balance get reduced by the amount I paid for. At no given time I will make a purchase without having a positive balance into my account, infact my account doesn't allow me to make a purchase in the event where there are no funds available.

Please note, I refuse to pay for any charges that are being accumulated from this account and I want you to close the account as I will no longer be using FNB anymore due to the reasons mentioned above.