IMO, you should be able to ban -any- noble that ends up in your prison, just at a hit to honour/prestige if they aren't already an infiltrator or rebel commander.
Nope.
You can ban a noble in your prisons who is a
criminal—that is, they were imprisoned in the act of committing some non-conventional act against your realm.
I think that this notion of criminality being required for a ban—which is essentially a death sentence—has been lost over the years since infiltrators were introduced, and it first became possible to ban nobles who were not a part of your realm. Yes, it
would be convenient to be able to just ban any noble that crossed your path, because then you could execute them out of hand if you ever caught them again, for any reason!
But that goes against BattleMaster's guarantee that death is rare, and pretty much requires you to do something to choose it, or at least choose to risk it. It is also simply not the direction that we feel is appropriate—and even if we do eventually push for more character death in the game, I do not believe that this is one of the ways we would choose to do so. It simply makes it far too easy to drive someone away from a realm.
