This is what I do when I don't care to have anyone else use my computer. I shut it off and unplug the power cord enough from the tower so that it won't turn on but looks as if it's plugged in. Then I unplug the other end from the outlet. Most people are stupid enough to not realize that the back of the PC is the problem, they just see the one from the outlet. Then when it doesn't start, they get frustrated and leave. Then I come back and get to yell at them for "breaking" my pc and they get all the trouble. Just make sure to pull the plug out of the back all the way so they don't catch on if they're sticking around. They'll be none the wiser.