If a student drops the course, you can remove them from your roster (i.e., "unenroll" them from the course) by navigating to the Students tab on your course home page, selecting the student from the list, and then clicking Unenroll from course.

Unenrolled students' annotations will not be deleted from your course, to preserve the conversations they already participated in. If a student later rejoins the course, they would simply enroll again with your course code (or by launching from your LMS, if you have integrated Perusall with your LMS) and they will automatically be re-added to your roster.