Students in my course may be dismissed or withdrawal before the end of the course. they have the option of retaking the course when it is offered during another session. Is their previous information cleared out with the new session offering including quizzes, disqus submissions etc.
If it's the same student account (i.e. email address) and course that the student is taking then their existing Disqus discussions, quiz results, etc will still be linked to the course. If you need them to start afresh, then you can always make a copy of the course for them to sign up to.
I hope that helps! Happy to help if you have any further questions at all.