Money being deducted from account without explantion

I am very dissapointed with Fnb, They added R3 865, 00 on my account. I called they bank and I was told that there is a merchant who didn't bank their money and they reversed it. This happened on the 12 of March. I didn't use the money. Money has been deducted from the account from that day until today. When I called the bank I was told it is because of pending amounts from merchants who have not taken their moneyI understood that because there was excess money in my account. Now everyday my account is on the negative I deposit money and the next day it's again on the negative. Now Fnb has deducted more than the excess amount that was R3 865. 00 and Im afraid of receiving funds with the account as they keep deducting money everyday. I called the bank but it didn't help