Force users to log off

I want to kick every user off the network at night. Users log on and leave files open for days at a time. The built in log on restiction hours stop users from logging on after hours but does not kick them off if they logged on during the day. I get users logging on and leaving stuff open for days, screwing up my back up and compliers and other utilites. What do you do to take care of this problem. Reboot is not an option.