So you actively promote metagaming in your game?
There's metagaming, and then there's metagaming.
When you first learn to ride a bike, you are consciously thinking about what you are doing until it you get enough practice and experience that you just do it without a lot of conscious effort.
RPG's are like riding a bike.
When you first learn how, you have to metagame a little bit as the GM and other players coach you on how to immerse yourself in the setting. You need reminders like "that's not something your character knows," or "a novice adventurer (i.e., low level character) would probably realize that obstacle is beyond them, or "sometimes you encounter monsters or NPCs that are just part of the flavor or setting, not meant to be dealt with on the spot."
If a player is clueless, you have to ask if it's due to inexperience and if so, just cut them some slack and allow a little metagaming until they get their bearings.
It's when an experienced player metagames, knowing it's a no-no, that it's a problem.