Bump, as it just happened to me too.
I have just had a clan leave my kingdom with 4 members all at 99 relation. They went and joined the Aserai, who currently have zero castles and zero towns, on account of...diplomatic relations we've had.
I'm trying to actually eliminate the Aserai, I manage to persuade their clan members to join me, they're at 99 relation, and a week later they defect to the Aserai again despite that kingdom having no settlements at all. How the hell are we supposed to win this game when we can't destroy a kingdom at all and clans pull this kinda crap?