Or just unplug the internal cable to the button. In real life, people love to force power off on computers. It's the one thing where they feel they have control.
I once shipped out four or five FreeBSD systems set up as appliances. No monitor, just a small form factor machine. They included instructions with pictures which clearly stated "do not ever force power off on this machine." No fewer than three of the users of those systems proudly told me that they routinely shut them down by holding down the power button until the light went off.