Forcing shutdown can be done easily by creating a scheduled task on clients issuing the shutdown command. I create these tasks with a simple computer startup script run via a GPO.
REM Delete existing tasks schtasks.exe /delete /TN Shutdown /f schtasks.exe /delete /TN Log_Off /f REM Create new tasks schtasks.exe /create /SC WEEKLY /D MON,TUE,WED,THU,FRI,SAT /ST 21:00:00 /TN Log_Off /TR "shutdown /l" /RU SYSTEM schtasks.exe /create /SC WEEKLY /D MON,TUE,THU,FRI,SAT /ST 21:05:00 /TN shutdown /TR "shutdown /f /s" /RU SYSTEM schtasks.exe /create /SC WEEKLY /D WED /ST 21:00:00 /TN defrag /TR "defrag.exe C:" /RU SYSTEM exit
As you can I see I also include a task to defrag the c: drive once a week.
I currently have moved away from this technique now and currently force clients to check the WSUS server for updates and install updates before shutting down. See my other article for details on this.