Another solution could be to use a third party tool which can be started from the logon script, runs hidden from the user, and which will shut down the PC at a certain time.Prior to implementation, you could warn all users via mail or something that the new policy for computer usage is that all logged on workstations will be automatically shut down at say, 22:00, and that if they leave files open, the changes will be lost. Kinda rough, but it will work...
This will acomplisch a couple of things: Any updates/changes/checks you run via the logon script will now be done every day, so no need to hunt down PC's that never log on but keep all sessions active.
Also, users are more likely to shut down their computer, not wanting to lose data, so security is better kept, and power is saved for the company...
There are at least a couple of tools out there that provide this function, and are freeware... 
