When directly attacking a mob, EntityTargetEvent will fire with TargetReason.TARGET_ATTACKED_NEARBY_ENTITY.

Based on the API docs, the correct result for this case should be TargetReason.TARGET_ATTACKED_ENTITY.

Steps:
1) Run fresh Spigot install with one plugin, using the listener below
2) Log in and use /summon Zombie
3) Punch the zombie

Result: EntityTargetEvent fires with TargetReason.TARGET_ATTACKED_NEARBY_ENTITY.

    @EventHandler(priority = EventPriority.LOWEST, ignoreCancelled = false)
    public void onEntityTargetEvent(EntityTargetEvent event) {

    	if (event.getReason() == TargetReason.CLOSEST_PLAYER) {
    		event.setCancelled(true);
    	}
    	
    	getLogger().info("EntityTargetEvent: " + event.getReason() + " " + "Cancelled: " + event.isCancelled());
    }

[SpigotTest] EntityTargetEvent: CLOSEST_PLAYER Cancelled: true
[SpigotTest] EntityTargetEvent: CLOSEST_PLAYER Cancelled: true
[SpigotTest] EntityTargetEvent: TARGET_ATTACKED_NEARBY_ENTITY Cancelled: false

It is necessary to cancel TargetReason.CLOSEST_PLAYER so that you can hit the mob before it has already targeted you.