Unauthorised deduction/debit

It seems that FNB decided to debit clients accounts left right and centre today!!! I thought I was the only client whose account was debited without consent for arrears on loan accounts but seemingly not. My account was debited for R2500 on 31 August without any prior notice or consent. If the account was handed over why not get the company to deal with the client - are you not paying them a service fee? Its another thing if the amount was reasonable - but R2500!!!! - the last time in May they debited R250 on the 5th of the month and i thought that Nimble arranged with FNB for the deduction - I thought it was a mistake today when they debited R2500 - if your account is in arrears and you have made an arrangement with Nimble for R250 and have not stuck to it - why not only debit R250????? My finances for the rest of the month is now messed up!!! So FNB can decide when and how they feel like it to just debit clients accounts for whatever amount...This is totally unacceptable- I need this Debit order to be reversed immediately!!