I would start by calling 911, and then I would have to decide from there. I have called 911 or our local non-emergency number quite a few times, as I live across the street from a park where kids and teens hang out, where teens pull up looking for a place to blast their stereos - I've asked a few to turn them down and then called the non-emergency number, and I have always been told not to deal with it myself - the police would much rather come than have a civilian involved... but it depends on the situation. There is no hard and fast rule that I follow, because there are always too many variables to be able to give a straight answer to this type of question.