Zachary solution is the easiest one.
Another option is disable the nextButton onPageShow… then re-enable it when needed.
I am not sure whether the disabled option is only in Lectora Online or also for Inspire/Publisher.
If its not, another option to disable a button (or whatever interface element ) is adding a blocker on top of it.
A shape or transparent png that shows on top of the clickable element and thus prevents the user from clicking..
Even an empty textbox can work as a blocker 😉 Noticing that often in delivered projects… 😉