Discussion of table top roleplaying games.
By setting expectations.
You're all adults, and you're all here to have fun. If you go and read the adventures and walk through everything with no surprises or challenges, then as the DM, I'm not having fun, and won't keep doing it. If you steal the limelight from other players with your "secret knowledge", they're not going to have fun.
So if you're going to read it, great, but use the power wisely, so as to not undermine the spirit of the game.
You don't. You just remind them that everyone is there to have fun. Since part of having fun is staying in character and taking on the adventure, if the player starts making choices that are obvious meta gaming like that, they just aren't part of the group any more. Or, if it's everyone, they can find another DM.
That may seem harsh, but it is possible to not only play while having read a module, you can even replay the exact same module multiple times. It just takes the players and DM being willing to do the work to compartmentalize the previous knowledge. That isn't always easy, and not everyone can do it (much less do it well), but everyone can help each other to stay on track without "cheating". I don't consider prior knowledge cheating tbh, but a lot of people do
I agree. People generally are mature enough to keep meta knowledge from tainting their characters actions. To clarify: I don't mean meta knowledge like HP totals if enemies or similar. Not using that kind of information would be against the spirit of the game (IMHO). But having a character act on information that is explicitly not available to that character (but may be to others).
Counter question, does it matter? If they have fun by knowing what's coming and being able to prepare for it does that make the experience worse for other players or yourself? If not then who cares let them have at it.
If yes then you need to address it somehow, the usual answer to these sorts of things is to just have an adult conversation with the players about expectations.
If you know your players and that won't work then you have some choices to make, again do you care enough to put in more work on top of already running the game?
If it's going to be an issue and they will abuse access to the module even after you've asked them not to, do you really want to play with them?
If the answer is still yes then I suggest altering things slightly so them having pre knowledge doesn't help, locations of key stuff, attitudes of NPCs, stat blocks/ monsters and enemy placement could all be changed pretty easily with fairly little work, I'd definitely rework the monsters at least, I wouldn't go as far as traps designed to catch them out though that's punishing the party for a player.
I homebrew everything these days, but back when I ran off-the-shelf adventures, I customized and arranged large parts of them as well. I didn't do it to catch cheaters, but it would be very good for that.
Anyway, I just told players not to read outside sources and be careful about what you search for. I said if you come across outside information once, I'll chalk it up to an accident, but if it happens repeatedly, I'll remove you from the group. I didn't feel the need to explain or justify that.
I had one case of someone searching a name to figure out the spelling, and they ended up spoiling themselves for a major character reveal. They were a good player and they regretted spoiling it for them themselves, and it never happened again.
Only play with people who can't read. They have to believe what you tell them
Just talk to them over the table about this spoiling the enjoyment. I had a version of this problem in my TOR table, where one of my players was playing an elf so he was looking up Tolkien lore through chatgpt so his character was appropriately knowledgeable, as an elf erudite should be. IMO this is awesome, but at one point I had to ask him to stop by clearly saying "hey, this stuff we are playing now is strictly from the RPG books, so looking it up will spoil some stuff" or "your character should know this and that about who these guys are". Works out great
What others have already said covers it I think, don't play with people who do that. however if you have a player who you don't want to remove from the group but also want them to stop reading the prefabs ahead of time, consider doing a prefab where you make it a requirement for all players to read it ahead of time, except that player. Don't let them know the others have already read it. It might even still be fun. Once that player realizes everyone is doing the optimal actions and that they all have likely read the book already, let them in on it. If you had fun, keep doing it that way, the dice rolls should still keep it interesting. Otherwise, the player reading ahead will hopefully understand the point and if not, nuke the party from orbit and put an ad at your nearest game book selling store.
Or just create your own. It is surprisingly easy and fun. Borrow elements from your favorite publishings. Try out some of the numerous random generators available online. Make shit up as you go.
My chapters I've made usually entail making a 'boss' character then going from there. Where do they live? How do they make a living? Who are their friends/enemies? What material possessions do they have? When do they meet the player characters? Answer those questions and you likely have most of the episode made and no player will be able to read ahead that way!
It's not always easy or free to access published material - a lot but not all Adventurers League stuff is on Scribd, and there's a lot of third-party content on DMsGuild that's just a little too obscure for liberators to bother de-fingerprinting. If you can't find a free copy of a title within the first couple pages of web search results then at least you've set the bar for spoilers at having to pay for them.
As someone who has played through adventures after having read or even DM'ed them... It's not hard to take a back seat to other players who don't know the surprises and twists of an adventure. Encourage your players to make a choice, if they must read ahead then at least don't trivialize the adventure for the players who don't.