Well, you could just rely on me destroying your corpses so horrifically there is no way to raise them effectively. Like when I dropped a castle on them. They were more "paste" than "corpses" which pretty much prevented raising. Since I use mostly terribly destructive magic to battle, this isn't unlikely. Any non-annihilated corpses would be intentionally disintegrated to prevent ressurection by your clerics, since we need your soldiers dead more than alive and a liability to our forces; Res' works on undead if they are willing, which your soldiers will be. A few spells and we have part of an army in our walls. No thanks, we'll be destroying any corpses not already destroyed. That a good enough IC excuse?