There is a sample that does what you want at this discussion:
https://community.articulate.com/discussions/articulate-storyline/free-sample-restricted-and-free-random-navigation-using-variables-and-triggers
Free navigation allows the learner to choose the different modules in any order and leave at any time. Restricted navigation forces them to visit the modules in order and leave only after completing all of them. Mixed navigation allows them to visit the modules in any order, but not to leave until all have been visited. It sounds to me like Restricted is what you want.
