There isn't an easy way to do what you describe. This version of Gameme is more suited for courses that start with a new course shell each semester.
To create a new shell, make a backup of your current course (Manage>Backups), go to My Courses and create a new course, choosing the backup you just created in the restore from backup area.
Or, depending on how may students you need to remove, you may be able remove them manually from the database. Scores are in the gm_user_scores table. You could search/delete by user id and course id from a tool like phpMyAdmin, or run some SQL.
For SQL, make a backup of the table, then run something like:
[green]SELECT * FROM AT_gm_user_scores WHERE id_user = 23 AND course_id = 7[/green]
If the data returned looks right for deletion, run
[green]DELETE FROM AT_gm_user_scores WHERE id_user = 23 AND course_id = 7[/green]
Maybe there is someone out there who could add a new feature for this.