Instructors (or any site owner) have several options for managing site members.
A Brightspace course site is generated from official timetable data (i.e. rosters, sections, labs, etc.) and will stay synced to that source information. Changes to student enrollment made via Classlist will be undone by the next data sync--students must drop courses through BrockDB and appropriate steps must be taken to have a site members removed. More information is available in the Classlist tool article.