Unauthorised amount debited.

I have had an amount thats debiting off my account over and above the normal vehicle installment and i have called Nedbank Durban Office to check what the amount was for and i was told its was for someone to come over at my place to check if the vehicle was there because I hadn't made payment for the overdue R2000. I requested for proof and date of this because I had made payment and had also sent confirmation of payment for this amount. I received a letter in December with the amount and i made the full payment of the overdue, yet the R700+ keeps coming off my account. This has been happening for 3 months and today i called my bank (FNB) to reverse the funds deducted without my authorisation, i was told Nedbank put a block on this amount and they can't reverse this amount. If i can't get proof for this additional R700, i will not pay it. For Nedbank to put a block on an amount that they are debiting off my account without authorisation, that's very wrong. Without proof of this amount, I will not pay.