The conditional navigation feature allows you to block access to specific screens.
To block the screens of your choice, go inside the editor, in the timeline that displays your different screens.
Click on the three little dots corresponding to the screen you want to block and click on "Block":
If you have three screens: screen 1, 2 and 3 and that you block the screen in the middle (2), then the users won't access the last screen (3).
Once the user lands on the screen that has been blocked, he can't access the following screens nor navigate back on the first screens.
To unblock a screen, click on the same three little dots:
You can also block/unblock all the screens at the same time by clicking on the lock next to "Screens":
This feature might be useful if you want to progressively unlock access to certain games.