I want to prevent users from typing anything on the keyboard, but still show the screen (so usual locking will not help). Unplugging the keyboard is not the answer I'm looking for, what other options do I have?
KeyboardCleanTool is a tool which 'locks' the keyboard. It can be enabled or disabled with the mouse.
MollyGuard is a small menu bar application that enables you to lock and unlock your keyboard and, optionally, mouse or trackpad with a hotkey combination. When locked, no input is accepted from the keyboard or mouse (other than the hotkey combination, otherwise you wouldn't be able to unlock your input devices).
I would recommend you KeyboardLocker for Mac. It fits perfectly to me.