You're fortunate. I had a room-mate who was mentally ill, a drug abuser, violent, and made direct threats on my life to me. I pleaded for help from the police many a time, but they would never hospitalize him. As soon as they showed up he'd sweet-talk the cops, lie, and they'd blow me off. When I showed them property he had damaged they told me they had no way of proving he'd done it. The only way, in California, the cops will arrest someone is if the person leaves marks on someone else. The only way to 5150 someone is if s/he tell the cops s/he is suicidal, homicidal, or is unable to feed themselves...and that's only if a person is in the 'right' neighborhood.

I finally left the situation (the room-mate), and in doing so lost most of my possessions. That was my only, and I mean only, option.

The laws in Calif can be very odd.

Rose's post is spot on, in my experience.