Difficult. I too have a problem with this sort of thing. My main one is people on the phone whilst driving. I want to buy a set of blue strobe lights that can be put in the windscreen when I see someone doing it, just to frighten the shite out of them, however I suspect this is highly illegal. I have shouted at a few people as I have driven past them whilst they are on the phone but what I tend to do now is give a long blast on the horn as I drive past them. With regards to the water bottle incident SC mentioned, I would have picked it up and put it back through his window gently saying 'I think you dropped this accidentaly', just to see what he said.