From wiki: https://forums.totalwar.org/wiki/index.php/MTW_Peasants
"One big problem with Peasants is that some AI factions tend to build lots of them due to their cheap cost. This hurts the AI's capability in combat and can detract from the game. Since it's not very realistic to have Peasants as garrison troops anyway, many players mod Peasants so they are not trainable, only available in uprisings and reemergences. It is generally agreed that this makes for a better campaign."
Does anyone know how to apply mentioned modification to the game? I couldn't find anything relevant on the forum and Google didn't help either.