Your English is fine, probably better than a lot of Americans.
I agree on all your points. I am playing as a vassal of the Western Empire. We completely stomped the Northern Empire. They have no towns, no castles. We have 60 of their lords as prisoners. Yet, they won't go away. I own Diathma and for the past year have been destroying/capturing small bands of their raiders. At any one given time there are 4 small parties of 10-14 soldiers raiding my territory. I wipe them out, four more spawn out of thin air.
Too, small Northern Empire armies keep appearing. Where are they getting the dinars? Where are they getting the troops? Where are they getting the supplies? They own nothing. And yet they have 18 clans, the largest number among all the kingdoms.
I have 50+ of their lords in my dungeon. I would love to execute them all, just to stop them from escaping and forming new raiding parties, but I can't. If I do, all of Calradia, even my own allies, hate me.
As you asked, how do you get rid of a Kingdom that is for all intents and purposes has been destroyed? As the game now stands, you can't. It feel like the AI is cheating. Which makes the game extremely frustrating the play.