Any block type labeled an unbreakable block will not be breakable by anyone but the owner. They will belong to whomever placed them. They can be used as building material i.e. ungriefable city walls, impenetrable vaults, etc. or protect block types from theft i.e. note blocks, fences, or anything that you want to be grief proof. You can right-click on the block with a tool in hand to view who owns it. To make block types unbreakable add them to the unbreakable-blocks list in the configuration.