Need help making a specific block type that can only be interacted with once by a player.

As the title says, I need some advice on how to make a block that rewards a player once per block of that type, scattered throughout structures. Basically it allows for multiplayer dungeon raids and keeps players from not getting rewards on multiplayer servers. The idea is that once it is clicked by a player, that specific block can't be used by that specific player again, but could be used by another. Example: Player 1 uses Reward Block A for Nether Fortress A, they're done with Reward Block A in Nether Fortress A, they'll have to go to Nether Fortress B to get the rewards again, this time from Reward Block B. But along comes Player 2, who then can use Reward Block A in Nether Fortress A independent of Player 1.

Any ideas? I've tried global variables, Goldorion's Array plugin, and player specific NBT variables, but nothing quite hits the sweet spot. It either locks all Reward Blocks for Player 1, or locks Reward Block A for all players, no real in between.