Do banks have a limit on overdrafts ?

Something,I personally hate and my bank continues to do so,even though,I have repeatedly requested for them not to allow any type of an overdraft,,, but my question is: Do banks have a limit on the amount of the overdraft for them to still honor the transaction ? If so what is it ?
The reason I am asking is my aunt plans on running an $800 overdraft on her checking account and repaying it with the accompanying fees/charges when she gets her tax refund check.
I just do not see the bank allowing it .

ps: please no speeches about it being unethical,that is not what my question is about !