You can do this by amending the Local Policy.

Obviously only do this if you are comfortable using the Policy Editor

Start - > Run - > gpedit.msc

navigate to User Configuration - Administrative Templates - Start Menu and Taskbar.

In here edit the setting 'Remove and prevent access to the Shut Down command' and set it to 'Enabled'.

This will also stop alt-F4 being used too.

You will still be able to issue the shutdown command via cmd

Kev