If you have someones belongings what are legal rights to them?

My brother in law stored some things in our garage(bed,dresser,shoes).He dumped oil all over our pallets of shingles and we asked him to removed his belongings.We tried to put all of his things by the side of the road but the police said we would be arrested if we did that because we were doing it so his belongings would be vandalized or stolen.We think we have the right to do whatever we want with the things.Who's right here?