Fiddle with the CMOS jumper
Partially unseat one of the RAM sticks
Lower the CPU thermal threshold to something like 45C
Change/remove the boot drive order
Put a floppy in the drive (if the PCs have them)
Change the Admin password using a bootdisk
Put a password on the BIOS
If the keyboard/mouse are USB, unplug them and plug them back into different USB ports. ON every Windows system I have ever used, this will prevent them from using the keyboard to log into Windows until the system reboots (so it is only good for one restart, but it slows them down)
In theory, if the drives are IDE and on the same ribbon, setting them both to master might do something, but I haven't messed with that in years.
What you want are nested problems. So, a couple of 'obvious' ones, preferrably where it definitely looks like you tried to cover your tracks. That way they will think they fixed it, and then BAM!, no they didn't. So yea, get them jumping through a series of hoops. None of the problems are insurmountable in and of themselves, but when chained together, may take some time to catch them all.
By lowering the thermal threshold you will force the system to restart even if there isn't much of a problem. If the systems are P4 Prescotts, then they should hit 45C in no time.