It's great that you've been looking out for other players. If I was one of the players you were "protecting" I would be extremely grateful. That said, pictures speak volumes. Just a small suggestion, next time take screenshots of the event happening. If you would have caught him with an open chest that didn't belong to him and doesn't have his permissions, that's concrete evidence and makes it really easy for a mod/admin to swing the ban hammer.
That said, good work. Hopefully the mods can gather evidence with the block history.