Years ago, traveling to Venezuela, it was brought to my attention the way the traffic authorities dealt with people from parking in the airport loading zone. The authorities would apply this sticker across your windshield. They did this because the Taxi drivers liked to park in these areas and enter the airport to solicit business from the people and thus blocking the areas with un-manned vehicles. These huge stickers stuck pretty good especially since the avg temp in Caracas is 95 degrees.
Anyway, I walked out with a driver one day, and the windshield was stickered. They even have a message on the stickers to not park in these zones. The driver grumbled, moved his car to a better place with his head hanging out the window to see, and started work to remove the sticker. It took him about 10-15 minutes and I could tell he had a lot of experience doing this. This included sopping hot water out his radiator and onto the sticker. No damage to the car/windshield after the sticker is gone. Those Venezuelans can be pretty clever.
Anyway, do you think it would be socially unacceptable to place a very small-scale sticker having a polite message on it, like maybe 4x6in, either on the sweet-spot of a windshield or even the drivers side window? I imagine if the owner caught you putting a sticker on their car things could get ugly. But what else could be done to get these guys to show some respect to our situation?