If you don't want your attendees to be able to see game challenges, you have the option of either disabling or hiding all of the challenges in your app through the Gamification area of Experience.
When you disable a challenge, it will not appear in the app, nor will entering the passcode in the app reward you with the points assigned to that challenge. Each challenge will have its own option to disable it, so you can choose which ones you want to make inactive by clicking into the challenge, and deselecting the Enable Challenge option.
Hiding a challenge removes it from view in the app, but entering the passcode into the Gamification section will still award you the points for that challenge. This feature can be used for games where you want to reward your attendees for organic behaviours, rather than completing a set list of challenges that they can check off a list.
Note: The hide option applies to all the challenges in your game, so there is no option to just hide individual challenges.