That is a question phoned into the California Department of Consumer Affairs regularly. Basically, the repo man can come and get your car, even go on to your property in many circumstances, but they can't break into a garage and they can't threaten you.
"Threats of violence, be it actual violence, be it gaining access to the car by damaging property, those types of things," says Jeffrey Mason from the Department of Consumer Affairs.
If you are on the scene when your car is being repossessed, you must be given a chance to get personal property out of the vehicle. If you aren't there when the car is snatched, you have 60 days to retrieve those items.
There is additional information and advice about bill collectors here: Confessions of former debt collectors