Fnb allow card payments when no funds available then charge you R100 per transaction without customers consent

On 23 and 25 November Fnb deducted R100 and R300 bankcharges from my Premier cheque account because of transactions they allowed when there was no funds available. I was not aware that there were no funds in my particular account. It costs R8.50 for a decline. Since when does Fnb allow transactions if there are no funds. I did not give authorisation for this. I can deal with R8.50 and a decline I CANNOT DEAL WITH FNB *****ING MY HARD EARNED MONEY and charging me R100 for allowing a transaction if there were no funds. My account is now overdrawn and when the secure chat is approached the agents refuse to assist me because my account is overdrawn but Fnb this is your fault!!! Fnb are ******* and ***** money! I did not authorise you to allow such transactions!