Hello, this is the first time that I write something in this forum. I think that your work is amazing, you are realizing something that I desidered from a long time
I really like this Property System, but I think that the easier way to avoid "griefing" is like in real life. Why we don't destroy everything that we see?(such as walls etc...)
I think that the answer is : because this is impossible! It requires a lot of time!
Destroy a stone wall is impossible everywhere with bare hands, and with tools (like a pickaxe) it require a lot of time (1 cubic meter of stone in 5 second?XD).
So, my ideas is that destroy blocks should be possible everywhere, with some differences:
1) In any land that isn't yours, you take(for example) 5 minute to destroy a block of stone with a pickaxe(I don't know if you want to put tools in game, I'm assuming a lot of things XD)
2) In cities there may be guards that attack everyone that destroy a block without a permission
3) In your lands the time required to destroy a block is much less that in other place
4) Without the right tool you can't destroy a block (except for some object like tables, straw roofs etc..)
In this way is possible to conquer/attack/destroy enemy cities, that aren't indestructible...
I think you may underestimate the time that a griefer is willing to put in to be a jerk.
Either it takes so long that no one will do it and it's essentially like not being able to do it at all... or it's easy enough that a dedicated griefer will wreck everything. If you make it hard but not impossible then the griefing will be even worse because now it's a challenge and a badge of "honor".