variables:
   {rewardblock.found} = false

command /setrewardblock:
  permission: rewardblock.set
  trigger:
    set {rewardblock} to targeted block
    set {rewardblock.found} to false

on rightclick:
  block is {rewardblock}
  {rewardblock.found} is false:
    message "You're the first!"
    drop 500 exp at the player
    set {rewardblock.found} to true
  else:
    message "Sorry, someone was faster"