Two checkboxes in the wrench menu labeled "Public", and "Locked".
Public bricks can be built on by anyone, but nobody can destroy, paint, wrench, or do anything else to them.
Locked bricks make it impossible for you to accidentally hammer them, paint them, etc.. They can only be unlocked by wrenching them and unchecking the "Locked" box. However, the D-wand can break them.
Of course you can check both, because there are many places you'd want to have public locked bricks. I just want to prevent confusion that will probably happen anyway.
The reason to have these settings is so you can build landscapes such as islands, mountains, or (in conjunction with maps such as Above) a bunch of connected baseplates to customize how large you want the building space to be.
No, nobody can exploit this.
Yes, people can spam things on them. People can spam anything anywhere. If it's tricky to D-wand large amounts of spam off of the bricks, just clear that player's bricks. If you say, "I don't know. It sounds like people can spam this." it won't even make sense, and you obviously haven't read the entire post.
So how does it sound?