Oh dear! This happened ONCE in my car. Only once.
I think I scared the you know what out of my child!
I screamed in a horrified way, immediately found a safe pull off, where I SLAMMED on my breaks in a forceful manner to get my soon to be made point across, and jumped out of my seat.

At this point she was already petrified and realized that she'd made a big mistake!
I threw open the back car door and gave her a good swat on the bottom. (we don't often spank- but life threatening issues are an exception for me) I then made it abundantly clear that it was the law that she say in her buckle, that I would get in trouble with the police if she removed her buckle and that it was there to keep her from crashing through the front car windshield if our car was in an accident.

I think I really scared her good!! As well as her sibling riding along next to her.
Neither one of my children have ever tried this trick again!!
And no, I would never normally tell my children that police are going to get them. I often tell my children how helpful our police officers are. This seemed like a good occasion to use their "official" position to back up my own policy of keeping your seat buckled.
It worked!
-Sarah