When trying to kill specific item entities with : kill @e[type=Item] {Item:{id:minecraft:<item>}}, a command block will come up with an error saying: You must specify which player you wish to perform this action on. If an OP attempts to use this command, they will be killed as if they just typed in: /kill. Removing the dataTag will cause the command to kill all item entities in the world. Replacing /kill with /testfor will cause the command block to recognize specific items without similar trouble, as shown in the screenshots below.