Buy your own camo game camera (you can find them for under $100) and hang it on a tree in a disguised location when you leave the lot. If you are lucky (and if the camera doesn't get stolen), you might have some worthwhile pics (although you'll have a lot of pics of ordinary hikers coming and going).
Sadly, it appears that the sense of this thread is that being a pathetic victim is inevitable, and there's no point in even trying to stop the folks that are harming other people. So protect yourself as well as you can within the bounds of the law.
Where is the closest police station to these parking lots? Another option is to buy a junker bicycle, park your car at the police station, and ride the junker bike to the trailhead. That's a pathetic option, but it appears that we've "given up" on trying to stop this type of crime.