Clan War Spies Need to be Dealt With
For me, it has always seemed wrong to deceive the enemy clan, ask to join, join, request troops, and leave WHEN that person is a loyal member of the enemy clan in clan wars. So the problem here is not so much clan hoppers, but the deceitfulness of these spies.
Perhaps this is a strategic element of the game that I was never aware of, but to me it just seems wrong and unfair, and I think that Supercell should restrict enemy clan members from joining while in war.