What to do kids always fight?

I have the some problem with my oldest soon. When it is too turbulent in the car I threaten him to leave him right there and go to home without him. Of course I would never do that, but it's enough to calm him. Sometimes when he don't listen I just go to a packing lot and detach him from his seat just to show him that I will do it (but as a said, I will never do that for real). Sometimes, threatning do the job. Have you tried something yet ?