Change their runlevel to 0, so it just reboots upon bootup. That drives those that don't know much about *nix nuts.
At my last job, to prevent users from searching job sites, we redirected anyone who went to the search page at www.monster.com
to a page that looked like monster's own but said their site was down for maintenance, funny how we actually had employees start to notice when it had been supposedly down for weeks and they actually sent trouble tickets on it cause they said they could get to it from home.. not a smart move, now we knew who was definitely looking for new jobs.
Use to have a perl script that would launch the X windows logo on a persons remote machine, whenever they clicked it, it would just spawn another image. This would drive them nuts as each time they click to close, it just opens a new one filling up their machine. Some never figured out how to kill it off, so they would just reboot. It was really funny when we would add it to pop up upon login.