i tend to herd them to a province not too rich. then attack a few times so their army won`t have a chance against the border garrisons.(leave 100 +1 alive!!)

they will either turn rebel itself, or get crushed by a other faction. but then the "returning heir"- party will be their problem first turn. the chances to marry a daughter from a faction you *pet* is rare, ..but there, especially in terms of "peace-talks" and their denieal..disagreement.

i use the pet for army testing, general- first-battle +1 star factory, and artillery range. and when another big party shows up, i can leave (garrisons don`t) the conquered soil and leave a buffer (..concerning the main army is "where the pepper grows")