I'm almost convinced that if I see a driver looking at their phone while moving (the absolute lowest standard of "don't use your phone and drive"), I should be able to put my U-lock through their window. It's only fair to add to their cost enough to offset the extra risk they cause me.
If you enjoy occasionally bullying idiots who drive with their phones, do it. The police won't care, and there would not be enough evidence to prosecute you anyway if you obscure your face with.
The person in the car won't understand why you've done it, and will get all uppity and defensive, but will probably be jarred enough by the assault on his or her personal space to make stupid mistakes throughout the day.