Please stop *****ing my money.

For some reason FNB has the habit of saying that my account balance goes below zero (which it never does because i never use more than my available amount) then charges me for being in the negatives. FNB charged me R200 for supposedly making purchases with an over draft account but i have a student debit account that does not allow overdrafts. So how is it possible that my account goes into overdraft even though it cant (and i didn't) then i get charged R200. Honestly, this inst the first time this is happened and i believe FNB are *******. The last time this happened they refunded me my money and apologized for the inconvenience but i'm sure if i didn't say anything they would have kept my money. ******* ******* *******