Hmmm... Royal dukes... you can now kick them out and force a secession, right?
Yep.
That's basically the same as banning them.
Close, but not quite. You can rig a ban with the proper circumstances to prevent a secession. Assassinations, wounds in battle, in prison, away from the city, etc., would all let you get rid of a duke without allowing the to secede or change allegiance first.
