I can not imagine going so far as to pull someone over. We (my partner and I) have honked the horn, or more rarely flashed the lights at a driver to get their attention when they are doing something stupid. They were situations where I would have honked the horn in my personal vehicle, like someone trying to merge on top of us.
We have only once turned on our lights and left them on (briefly) because of an erratic driver...and that was because the guy was going the wrong way down a one way, right at us. After we turned on the lights the car turned (the wrong way down another one way) and we turned them off and called the guy in to our dispatch with a partial license plate. Unfortunately about 5 minutes after that the car entered the freeway on an off ramp and killed himself and 2 others in a head on collision.
We did wonder what would have happened if we hadn't turned on our lights, but in the long run us seeing him pulling out of the bar parking lot into the street gave the police enough evidence to go after the bar pretty hard.