Currently, custom block drops can be dropped whenever the player breaks the right block (under the right circumstances). But for some purposes, this is undesirable. It would be nice to add a new option to custom block drops that ensures it won't be dropped if the block was placed by a player.

This will be quite complex to implement because we will need to keep track of which blocks were and were not placed by players. Simply remembering all coordinates where any player has placed a block might drain too much memory...