When you have the login screen present you have the ability to shut down, sleep or restart your computer from the option below the users. If you want to remove this option you can. This means users can only login and don’t have the ability to shut down the computer. This quick change is done from the system preferences and allows you to customise the login screen to suit.
To disable these options open system preferences and select the Users and Groups tab. From the list on the left hand side select the Login Items. If the options a greyed out use the lock in the bottom left to select unlock the options.
From the main options deselect the option “Show the Sleep, Restart, and Shut Down buttons”. The will remove the options from the login screen.