I don't run a lot of pre-published stuff, but when I do, I will specificly ask players to not read them. Mostly our group is honest (We also nix looking up a monster in the manual durring a game, though not realy a problem either.)
However, if you do have players who do that, you probably know who they are. Make some minor changes in the dungeon (move a secret door, add a trap, swap the contents of an entire room) to throw them off. If they make it obvious they have been reading the manual/adventure ("I rolled a 20, and the module says there's a secret door right here"), make it very rough on them.
Personaly, I won't tollerate players metagaming by looking up info they wouldn't normaly have access too, and subscribe to the theory of spontaniously giving them different and deadly powers if I catch a player doing it durring an encounter (only happened once at a table I played at, GM was not happy, I ignored everything read out loud). However, I have no problem with reading about a monster afterwards (especialy if they won't encounter another). I simply ask that the players downplay that knowledge.
I have to re-read most of the monsters. I haven't run much since 2nd Ed, and intentionaly did not read much into the monster books to keep the mystery alive. Sure, I might know Trolls regenerate, but I might either play it more as hersay (perhaps the character thinks fire/acid is simply more effective and will not even attempt other damage) or try to look for what is effecting the creature (I have no problem giving a player a general idea how effective a blow is without actively telling them DR, Resistances, ect, prefer the mystery).
odd adventure names (might actualy be other modules, but should throw players off)
Archon's Advocate
Revenge of Bahna-Rhatu
Blood of the Halflings
Tomb of Suplimental Evil
