Given that everyone seems to be in confusion over this I thought i'd propose a rule change, or rather, creation of some rules, since frankly the one that is there was designed for a time before PvP was even possible. It was a rule designed to protect people's work, since theft simply wasn't an issue. Here's the existing rlue, and the proposal.
Current Rule:
Proposed Rule:1) No destroying anybody else's buildings without their permission, period. If by accident you destroy a block that is a part of a building you should replace it.
Spoiler Alert, click show to read:
There. My reasoning for these alterations is simple. I do not believe it is right that simply burying your treasure beneath whatever block you fancy should make it impregnable simply because of griefing rules, especially if those blocks are then replaced. Another reason is that if you ruled that it was griefing, it quickly becomes very unenforceable and unworkable. Any good thief would simply do it when no-one was looking. Likewise, any such rule would effectivly outlaw stealing/looting in PvP. As much as I would like a situation where treasure is protected via cunning traps, there is no way to prevent someone simply blocking off their resources, and so any rule barring making such resources unreachable would be completely unenforceable and simply put those who did follow the rule at a disadvantage. Lastly, it's perfectly possible through the use of bland/remote locations, decoys, and/or well-designed traps, to add in enough of a challenge in stealing your resources in the first place.
The last bit was just to declare that such things are deemed illegal, even if it is incredibly difficult to enforce.
I'll allow for some discussion on this before putting it to a vote.
Edit: Added one last line, since nothing in the rules actually states about stealing AFAIK.