I want only root/admin to be able to shut down the system. How can I do it in GNU/Linux, specifically Ubuntu, system?
This is generally the norm for most systems, but Ubuntu is a bit different. Here is an article with all the steps for Ubuntu.
Normal users, i.e. ones without physical access to the system and/or root, can't shut down the system.
Once you give users physical access you have to assume that they can get full control. Shutting down is done with the power cord. Breaking in is done by booting from USB or the DVD drive.
You can remove the DVD drive, but USB is harder since that's also the keybard and mouse.
You can, of course, lock the case down, password protect the bios, epoxy the keyboard and mouse in place, and fill the extra usb plugs, etc. It does give a impression that you might not trust your users though.
You could also look into the linux terminal server project since opening a remote session will not let you do a shutdown.
remove those users from the sudoers list. i.e. /etc/sudoers
for reference see http://nixcraft.com/linux-software/12261-etc-sudoers-file.html[link text]