An easy fix they could do is that you can leave a guild in a war but can join another until XX days or until the war is over with the current guild(s).
So the kidnapped players could leave and play solo.
what IMC does have a feature to actually betray your guild which would mean not randomly recruiting random players. I actually want more drama now than the consented warring.
On Granado Espada, you need to wait a week before you join another guild, if you leave one.