First of all for me to use my firearm to "take him/her out" will require what I percieve as a clear and immediate threat to innocent life. The "gravest extreme" requiring immediate remedy. In that event a sign on the door of any kind has no meaning to me at all and will be the least of my worries.
Your life is not in immediate danger because the robber is pointing the gun at the cashier or other customers and you know you can take him/her out from your position. would you do it knowing it's a "no CC" zone?