I'm using Gnome and I'd like to still have the ability to reboot/shutdown from one particular account as well as root. How would I modify the chmod command to add this ability?
Read up on the use of sudo
which will allow certain users (you define them) to use some (and you get to choose just which) commands that need to be run as root.
Or, if it is just reboot/shutdown, you could create a new group called reboot
then you could add your special user to the group reboot
and then set the permissions for reboot shutdown
to 750 and then alter the owner:group
of those executables to root:reboot
That way, those commands can only be run by root, or a member of the group reboot
The power button is more complicated. Basically, if users can get to the power button, or the wall socket, they can do as they like with your PC (Eg boot to a live CD). You might have set your CMOS not to allow this, but if they can access the hardware, they can remove the CMOS battery and then make it boot without a password.
That's why banks, and others, spend a lot of money to keep their computers in "secure" buildings.