I do think that there should be a way to remove Royals from the realm. However, I don't think that simply removing their "unbannable" status is the right thing to do. Honestly, I think what should be done is to rework Exile. Yes, it's meant to be a battle of wills, but as it stands, there is effectively no incentive for someone who wants to see the ruler hurt to leave the realm. Yes, they lose honour and prestige—but if it's essentially a choice between that and just losing, leaving the realm in ignominy, what the hell does honour and prestige mean?