What should I do about my neighbor?

My 9 year old son has accidentally thrown a hard ball into my neighbors yard while playing catch with his friend a couple of times. After my neighbor confronted me about it I told the children only tennis or softer balls will be aloud. So one night I took my son out to play catch and we used a regulation little league ball that my son uses in his. To avoid him throwing it over that fence I threw in that direction knowing I would not. My neighbor freaked out saying we couldn't use that ball back there and I assured him the ball would not go over the fense he continued to call me stupid and did not want to reason with me. So now my neighbor is folding beer bottle caps and throwing them in my yard. I would assume for someone to get hurt on. My child threw the ball over purely by accident and this grown man old enough to be his grandfather is purposely trying to hurt someone. What should I do to resolve this problem