You could also use a hidden power switch. On my auto I have a switch hidden that shuts off the fuel pump. Even if someone breaks in they won't be able to steal it.
I would think you could use the "turbo" switch since most don't use that to disconnect the power to the HDD. If that does't power up it won't boot or be accessable. It would be easy to hit both ON and turbo at the same time so they don't catch on.
I have never done it so you would have to improvise on your own.
How about a relay so that when they turn it on it sends a jolt to the chair? Or sprays them with pepper spray? Put some of that "stop chewing fingernails" stuff on the keyboard and wait till they pick their nose or rub their eyes.
Or get new "freinds".