I was referring to "get rid of the knights that do not support you before you secede", but I think your suggestion makes sense.
As it is, there is no way to get rid of them at all.
But something we must live with until such a feature request to change them is implemented. It might take a long while, but if you don't adapt and deal with it until such a change occurs, then you're intentionally subverting your own efforts.
They're like laws, you know? We might think some of them are unjustified, unreasonable, and outright wrong, but until your legislature officially changes them, you must follow them.
So if the law says women aren't allowed to drive, and a woman gets arrested for driving, your first reaction is not "what the !@#$ is this stupid law?", but rather "she shouldn't have driven!"?
Yes, people should prepare to deal with how the mechanics work. And in his particular case, his secession was done poorly. However, that does not mean that nobody should be able to kick nobles out of their duchy one way or another. The context of this particular case does not change the fact that dissolving one's oath should be a legitimate way to remove someone from your duchy, so that before a secession occurs one has the possibility of weeding out those that might protest him.
I'd add that I don't think council members having gotten power through a vote or by secession should be protestable for 5 or 7 days either. Vassals are supposed to support their lieges, through misery if suffering if need be, and as such protesting one's duke out of office is quite anti-SMA.
