We had a lot of fun with this in University 20 years ago. Our computer lab had a handful of shiny new Sun Sparcstations. The freshmen would regularly end up hogging them (no timelimit) and we needed them for project work, so we'd just remote login to them and cat some pre-recorded messages out to /dev/audio. A very official-sounding "Please log out now, system going down for maintenance" usually worked wonders, although occasionally more serious messing with people's minds was done. Some of the 1st year students had literally no idea what a computer was capable of and would believe anything that came out of its speakers. :D