Why bother? I have the same experience as you; most of the other factions refuse to make peace. Those who do make peace generally attack you again. So, if I don't want to conquer them, I turn it into a training ground. I destroy the bulk of their military, take some key provinces, but make sure to leave them enough territory to be viable from an economic standpoint. Then I leave an army behind to destroy whatever forces they recruit to attack me. I get veteran troops from it and gain experience for my younger generals.