Back when I was working on kiosks (I was on the engineering team) we would have a metal plate made to only show the keys required for the kiosk app. Some thought they could deal with it in software but after too many failures the old way or fix returned and stuck.
You can also glue the keys in place if you don't want to do the usual cover.
I am using Samsung All-in-One DP700A3D-K01US as advertisement kiosk. It has various hardware buttons. The PC runs an app in assigned access using Windows 8.1, so user can't navigate to other app or start menu. But the hardware buttons offers various function (see image: http://i.stack.imgur.com/evhZE.png). So is it possible to disable it using ANY way?