Well maybe instead of nobling (complete take over) they could do like a game Knightsdivine (no longer around) in which the conquer could tax the person up to 10% of thier income. So maybe the town owner would have to pay 10% of his earnings to the conqueror? Say if he got paid $10, he'd only earn $9 if he was being taxed all the way, with $1 going to the conqueror.
Or ...
Like in Battleknight, in town vs town wars there could be an option to raid OR capture a town. Raiding gets money and such, but if they choose capture, and they win, then they get the town ... but if they already have one, they lose their old town.
As for a special unit ... I don't know ... maybe someone like a Federal Marshal? Preacher?