That would do far more harm than good.
Imagine keeping a player in your clan for up to 2 days who no longer wants to go there. You'll probably find a lot of Goblins in your war requests.
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I know its a game, but its more than frustrating when clan hoppers do that, you never know when its coming. It sucks but I guess thats what makes the game a challenge. Trying to keep good players can be a challenge I guess. But I wish we could stop the war when people leave the clan or at least the game should prevent them from leaving once in war.