People opening their doors on your car in carparks.

What is your general response to someone opening their door on your car and/or how do you avoid it happening in the first place?

Last weekend I parked in a populated car park where the bays were pretty narrow. As I got out, a little girl was about to get in to the car beside me and I could see what was about to happen so I stood there and watched. Sure enough, she opened her car door and slammed it open on the side of my car.

She was young.. about 7 ish so probably didn't really know better but it left a ding in my door. Her mother was on the other side of the car putting a baby in a car seat but she saw what happened.
I spoke to the girl for a couple of minutes and told her that it's not OK to open your car door on someone else's car door. Her mother didn't scream and shout at me so seemed ok with me gently telling off her daughter, so that's a plus I suppose.

I didn't really pursue it because my car is covered in dings and scratches anyway.. and then I realised... all of those dings and scratches are from other people in car parks.
Almost every panel has a dent or a scratch. There has to be something that can be done to avoid this, surely? What do you lot do when you don't catch people in the act? Is there some really obvious answer to this that I haven't realised?