If an end user has accepted a meeting invite, then decides they cannot make it and deletes the meeting instance, it will reappear on their calendar because Office 365 looks for any inconsistencies across calendars and tries to repair them. Likewise if a meeting organizer can no longer attend a meeting and deletes it, it will again reappear on their calendar.
Solution: The appropriate way to handle these situations is to either decline the meeting as an attendee, or cancel the meeting as the organizer. Any calendar entry with multiple attendees must be handled in this manner, or the Calendar Repair Assistant tool will recreate the meeting instance on their calendar.