I'm going to ask a guy to stop throwing fireworks under the cars
Why do you want him to stop?
I hate the fireworks and I think it's stupid to do them under the cars.
It is rude to destroy other people's property. If you think it's safe you should go and get them to stop -- otherwise call the police before they do some serious damage.
Whatever, I'm not going to do that .
Not sure what changed your mind but you or the 
police should confront that person and stop him from damaging the car. or worse. It's not ok to ignore something that could result in property damage, serious injury, or fire.