Hopefully, the neighbors understand that your children make their own choices and do not judge you for their behavior. I'm sure you did not tell them to throw rocks at the truck.
I think all you can do (since they don't want an appology) is to discuss with your children, at an age-appropriate level, where it is ok to throw rocks and where it is not ok. I don't think you will ever get them to stop throwing! Tell them to throw onto the ground or in a puddle or through a hoop but not at things or people.
You could also have them 'write' an appology note. You do the actual writing and ask your children to dictate it. You , of course, will lead the conversation. I think your 6 year old will understand what he is doing and why he was wrong. You can give it to the neighbors or not. The important thing is that your children learn that they were wrong, not how the neighbors feel about you.
I'm glad the police didn't come over. You want them to think the police are their friends and there to help them not scare the pants off of them!
R.