In Debian or Ubuntu minimal disabling the power button is quite easy. Just remove the contents of /etc/acpi/powerbtn.sh. That doesn’t work in Ubuntu with Gnome/Unity. The shutdown procedure is quite complex; acpid, gnome-power-manager, polkit all play some kind of role in it. After a lot of reading I stumbled upon a post in the ubuntu forums where someone kindly analysed the Ubuntu shutdown button proces: http://ubuntuforums.org/showthread.php?t=2020630

What I did to disable the power button completely, was eventually very easy: