I'm pretty sure a Duke can secceed if he's banned.
Also if the inactive Duke is such a problem, you can get rid of him by banning him. That it's never done has more to do with the player base than with any coding I think. And an inactive Duke often doesn't provide funding to a war etc, so that would be enough reason to ban him if you want. If you ruler/judge are unwilling to do so, perhaps rebel in stead.
I think many of the issues here are player based, not mechannically based. Protesting Dukes just breeds exploits though and I will restate my extreme dislike towards this option, despite my agreement that we should all try to make things more lively, there are better ways than adding protesting though.
