mawwbox i have the very thing you are looking for. we used this in our computer class to fck with someone who wouldnt shut up. lol
idk how your systems are set up, idk if everyone has their own pc or if you have workstations. or if this is an office environment or if the computers are in your dorms or wat. but assuming its workstations in an office then your golden. what your gonna do is when this person isnt around gain access to the machine he typically uses and install this program on it. poweroff download its called poweroff and its a work of art. it runs in the background and you can log into it over a lan and do things such as shut off monitors, turn off the computer, lock the computer, log the user off and much more. it should only take a few times of this before he gives up and thinks theres something wrong with his pc.
if everyone has their own personal pc, or the pc's are in dorms or the like, then it becomes alittle tougher. message me and ill give you a brief walkthrough if that is the case. cuz this is getting pretty long.