Your party members have the same experience, and thus achieve the same level as you do. Therfore, to prevent them from surpassing level 20, simply cease leveling them up after 20.
Do note however, that you need to be at least level 15 to obtain a prestige class. Also, holding off levels could offset certain party member banters/interactions.
I know that lack of game balance sucks Zach, but so does metagaming.