The easier way-->  Just put a 3 second press hold on one of the ten key buttons that takes you to the page you want, if it is held less than 3 seconds, or any other button is pressed it takes you to a big red page for 5 seconds that says code is not valid.

All you want is for patrons or others not to mess with it. if they  don't know what key or how long to press it, they cant get in, most people will think they have to enter a 4 digit code or something and give up right away rather than wait 5 seconds each time they try to press a number. This is simple and changing the main link button to a different button is easy.