It's hard to say without seeing it, but from the way you describe it, I think you want to use layers or slides instead of lightboxes. Lightboxes are set up so that the learner can access that exact same content from several different spots in the course.
This discussion has a sample that may give you some ideas about restricting access:
The Restricted Navigation does not show the link to the next section until the previous section has been viewed. The Mixed Navigation allows the various sections to be visited in any order, but does not allow going to the End until all sections have been viewed. The Free Navigation allows the sections to be visited in any order, and the End to be visited at any time.
