Presentle local elevator codes would require a hall button at the main floor. Most card reader systems are placed within each elevator to inhibit access to upper floors. Placing a card reader only at the main floor would not stop someone from entering a car when it arrives at the main floor and door open to let people off.

Any carder system for an elevator essentially keeps honest people honest. There are too many ways to by-pass the system, either by code required overide operations or by maintenance people.

An architectural barrier is a better choice or a US Marine with a fixed bayonet.