So, to be clear:
- Any noble can be executed if they are in prison in a realm from which they are banned. (Or if they're in prison as a rogue.)
- Any realm can ban one of their own nobles any time (through the Judge's normal ban page).
Those two things, together, give you all you need to execute a noble from your own realm in your prison. Just remember to ban them through the normal ban process, not the ban-and-release process.
More generally:
- The only way to ban a noble who is not from your realm is if they have been captured red-handed for a criminal offense.
- Banning a noble in this manner will release them from prison, whether or not they are a member of your realm.
- You cannot ban a noble who has been captured for any reason other than a criminal offense—even if they were identified as the perpetrator of an offense, but not captured red-handed, and then captured by patrols later in the same day.
The in-game mechanics are cryptic, confusing, and often contradictory. The circumstances in which some options appear or fail to appear are odd, to say the least.
I hope that this has cleared up some of your confusion regarding these mechanics. If you have other questions about them, please feel free to ask (though if it's not closely related to this issue, I'd ask you to please make a new topic for them). Not only do I want to make sure that people understand the game mechanics, if there are mechanics that are just gratuitously contradictory or confusing, I want to make them work more clearly.
(For instance, I'm now strongly considering creating some sort of feature where if a noble is identified in a crime, they will be marked as bannable if they are captured within the next, say, three days.)
