I was raised to not take or touch something that isn't mine. But jeez there are a lot of unknowns here. You are "assuming" he or she stole them. Maybe the owner of the lot said to use them, maybe he thought they were just there for people who need them for chocking. He certainly is not hiding them.
At my sons boatyard, bricks and blocks of wood are behind many stored boats. If you need them,help yourself. If it bothers you so much,talk to him. The problem is that when we "assume" it makes an a** out of u and me.
That's when we try to get back at somebody for what we think they knowingly did and things escalate from there. Don't jump to conclusions.