I affraid there is no way to remove only unenrolled user data from active course. This is very old issue with Moodle and new Course Reset option is not fixing anything as it only back-up/recover course without ALL users data. Pity such bug got into Moodle and stays in code for years and years if not decades.
Any good news on this issue? I have courses from students of different institutions and different schedules. So the possibility of deleting their submissions based on their enrollment status or their cohort would be fantastic.