You need to either set up user-level accounts for them to use, or just lock down the system so they can't access it without you there. Unless people are willing to take the initiative on their own to learn good security practices user education just doesn't work, so you need to employ technical measures to limit what users can do.
This. If you just want them on your comp, there's no "fuck you" like drive level TrueCrypt encryption. That's how I told my ex, "Use your own fucking computer, and quit trying to sneak a peak at my stuff, you sneaky bitch!"
That went over just about as well as you'd expect, she's still butt-hurt about it. But if you steal shit or fuck up my stuff, too bad.
She used to try and peak at the password until she realized it was over thirty characters of gibberish.