How about imprisonment. You could lock up ppl into your dungeons and demand payment for them with your diplomats, if you wouldn't get payment right after battle. Of course some of them would die in your dungeons over time and so on. However it ridiculous now how other side won't pay for their king or heir. "No we need no king, just execute him. We have nice assortment of adopted kids, who can take his throne." With impisonment your king would lie in dungeon untill dead or freed. :)