Apple - How to lock the keyboard in Mac OS X, but not the screen?

KeyboardCleanTool is a tool which 'locks' the keyboard. It can be enabled or disabled with the mouse.

Try MollyGuard:

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.