In System Preferences >> Accounts >> Login Options, we have the option
Show the Restart, Sleep, and Shut Down buttons.
I was wondering how else will we be able to restart, sleep or shut down the computer if these buttons are disabled?
Simple answer: You can't (sleep, restart, turn off)
The option is there to set the Mac up so other people can't just power it down or restart it. That's why you need administrative privileges to actually change the settings. Btw: The keyboard shortcuts don't work either if the buttons are hidden.
It used (?) to be possible by typing
Of course if you have physical access, you can still hold the power button or whatever.