Is this possible? I would even go for a restriction to former realm members. I find it interesting that a noble who betrays the realm when they leave is immune to banning, deportation and execution so long as they do not do any illegal acts afterwards. A duke can convert his entire duchy to the enemy realm and if arrested later in battle, the 'enemy of the realm' can't be touched. In reality that noble would be executed on the spot. Or after some torture.
It used to be that leaving your realm would automatically generate a ban. I don't recall why this changed.
