Tips for managing inactive users

I work for a small nonprofit that's interested in purchasing Prime to provide training to our clients. Ideally, we would like users to have access to courses for a limited term--say, 4 months. Would any Prime users happen to have some suggestions for managing users who may or may not take subsequent courses? I understand that once a user is deleted, he/she cannot be added back. But given our particular use case, it doesn't seem wise to delete users upon the conclusion of a course because they may return later. Thanks!