In my current game so far I've put down a few civil wars (playing as England currently). At the conclusion of the war, the vassals involved are all imprisoned, and I'm left to figure out what to do with them.
I've recently taken to revoking a title, and then releasing them from prison. In the case of a former Duke who is now demoted to Count, I'll most likely transfer his vassalage to another vassal that has a high opinion of me for obvious reasons (and I hate the colour red and negative signs).
However there are some cases where a Duke has multiple Ducal titles for one reason or another, and obviously revoking one doesn't demote them as previously described; since, as far as I know, I cannot make a Duke the vassal of another Duke, I'm stuck looking at this scowling face all day, to say nothing of future plots and factions this now wholly pissed off duke may start. The only positive benefit is that I gain the opportunity to spread around the Ducal titles and prevent one person from hoarding them all.
From what I can tell, my options are slim if I want to avoid gaining negative opinions of my other vassals. For example, I cannot execute them without gaining everyone's eire, nor can I banish them for the same reason. Alternatively I can release them, but they may already have a significantly negative relationship with me that cannot be resolved by releasing. I suppose I can let them rot in prison for ages, and admittedly I haven't tried that solution yet so I'm uncertain whether there's any negative or positive benefits for that solution.
In short, after successfully crushing a major uprising, what is an effective way of dealing with the imprisoned vassals (or asked another way: is there an effective way of dealing with them that best helps prevent future uprisings and plots)?