I know I've had to defuse a situation were a member of my alliance, who is no longer, took for granted the fact we help each other.

He proceeded to attack and abandon the valleys of a member, bigger than him by a lot, of an alliance bigger than us.

He manage to infuriate the player, who was about to call his entire alliance down on us, as my former member had teleported away. Now I have played the other end, and attacked without mercy, but either way, if they don't feel any wrongdoing or are just plain stubborn, they need to be beaten viciously about the head and shoulders.