I don't think you can do that with the power button but you can change how the shutdown button behaves or create a shortcut to just Shutdown and Logout from the desktop and or with a hotkey.
Right click on your desktop.
Click New then Shortcut
Type the following
For a Force log off
shutdown.exe /L /F
For a Log off with timer - Substitute ## in the command below with a number between 0-315360000 for how many seconds you want the time-out period to be before log off
shutdown.exe /L /F /T ##
For a Log off with timer and comment - Substitute Your comment here in the command below with a shutdown message up to 512 characters within quotes that you would like to see in the pop-up dialog when using the shutdown...