Stop third parties automatically populating calendars
Currently according to Microsoft it is acceptable for a 3rd party who you have not shared your calendar with to populate it with a request, this leads to unwanted Spam appointments being booked provisionally and makes the calendar get clogged up with spam appointments.
After spending days now with Microsoft trying to stop this , the answer is "events will populate the calendar is a normal behaviour of Outlook in Office 365"
Simple solution would be to stop all 3rd party invites from automatically populating the calendar and only allow users that you have shared your calendar with add an event. If you accept the event / meeting via the email notification then it is put in the calendar.