You say this like it's a threat of punishment.
But if you want to play a puzzle game, then you need to set puzzles to which the players don't already know the answers. I can't see how that's not obvious.
Punishment? I guess you could call it that. Not obvious? Oh, it is clearly indicated to the players before hand. There is no "ha ha, gotcha!" here. In fact the rules I most recently used explicitly state what skills are needed to identify various monster types.
How do you expect the combat with trolls to unfold if players know that trolls are vulnerable to fire, but you want them to play their PCs as ignorant? Do you expect the player to allow his/her PC to be killed by the trolls, in the name of roleplaying the PC's ignorance? If not, how do you envisage it unfolding.
Well, assuming I am running a game where (1) there are trolls, (2) trolls and their weaknesses are not common knowledge to whatever civilization that the PCs are part of, and (3) the party hasn't previously run into trolls and learned all about them, then...
First, any PCs with the appropriate skill can roll to see if the recognize the troll. If they roll well, then I tell them they know what they are dealing with, what the weaknesses are, etc.
If they fail the skill roll, then I let them know they see "Large green humanoids" that they cannot identify. It is up to the PCs what happens next. I rarely ambush my players, so there is a good chance that if they are running into a new monster, they will have options to avoid or retreat. Maybe they decide to go back to town and research it. But, assuming they have somehow got themselves into a combat situation, then after a few rounds it will be clear that the creature they are battling has incredible regenerative capabilities. What happens then, again depends on the players. I would probably allow the players to say that they try to burn it, because trying fire is a pretty common thing no matter what the actual weakness might be. Or I might give a simple intelligence or perception check, that could give them some clue ("You notice that the creature is shying away from your torch even when it attacks"). Or they might just run away (again to try and research).
I actually had something like this happen in a game a couple of years ago. The PCs did know what trolls were and what could harm them, but they were not prepared but started the fight anyway (they were trying to rescue a comrade who had unwisely started drinking with trolls and passed out). They fought and chopped limbs off the trolls, but they kept reattaching. The group came close to getting the trolls disabled long enough to rescue their friend, but a bit of bad luck and the trolls were recovering quickly while the PCs were not. They ran to lick their wounds (and get some proper gear/spells).
Edit: I wanted to add that I don't think I have ever run a game where trolls are unknown to the players.
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