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!