Why would you want to do that though? It seems a complete waste of time spent unlocking and levelling it. If you don't want to play as that class, don't play as it. It's as simple as that (and to ensure you can't, remove all of that class's weapon from your inventory and Armoury Chest - really, the class is the weapon and not the exp shown on your character sheet. Without that weapon or tool, you can never be that class.