People doing that are doing it intentionally, and they aren't going to follow your rule.

People who are open to listening are not pretending to be surprised in order to put somebody down. They are actually surprised and (perhaps) unintentionally hurting somebody. If that somebody is hurt, they need to ask themselves which hurts more, having somebody surprised you didn't know something (aka they think you are smart), or being unsurprised you are ignorant of something (aka they think you don't know stuff).