It's not as simple as you're thinking it to be. Well, it is, but...
You can lock doors, chests, and furnaces. I also think that you can lock crafting tables too, but I doubt it.
You can either lock them to open only to you, or you can lock them to only open to a certain password...
the lock command (as you probably already know) is /cprivate (you-only) and /cpassword <password> (password protected).
However, I don't think that individual blocks can be protected, so if griefers can get in, then the lapis will be stolen again