I had a drug addict best friend, and my younger sister was a horrible thief.
I remedied the younger sister problems by making sure I put all of my stuff away in my room and installing a heavy duty bolt lock on my bedroom door.
I ended up having to do the same with my bedroom at the apartment I shared with the tweaker friend once I moved out on my own the last two years of high school. Eventually I threw her out and changed the locks on the doors, but we kinda lived in the ghetto and she managed to get in by sneaking into the attic and wiggling her way through the large furnace vent in the kitchen. Yeah, I know...
I came home and heard her moving around in there. Buzzed on the intercom, and like a dumbass she answered it, so I parked my car around the block and called the cops to report a break-in in my apartment and that the person was still there. Heh...
I didn't stay long after that incident. My part time moonlighting job ended up netting me a few grand a week just from scheduled regulars so I moved into a gorgeous 3,000 sq ft townhouse in a beautiful gated community with my dog and never had to deal with people and their crap again.
As for my husband, he has his things and I have mine. The only people in this house who mess around with other peoples stuff without permission is the cats, and they're obnoxious about it. I think i've had at least 20 ponytail elastics go MIA in the last month, and when I was vacuuming I found nearly 10 of them, chewed up and hidden in one of the caves of their 9 foot tall cat jungle gym thing. You can't hide stuff from them! They steal earrings, socks, nail clippers, my tape measure, my wedding set when I take them off to wash dishes, wash cloths. The list is endless.
Get a P/T Friday and Saturday night gig as a dominatrix. It's recession-proof, the tips are awesome, and then you could save up to get your own place! Problem solved!