When you reject a candidate, scheduled interview are removed from their profile, but not from your external calendar. You'll need to go into Google, Outlook, etc to delete the interviews manually if you want to avoid getting notifications for them in the future.
Pro tip for Google Calendar users:
If you've connected your calendar using the Integrations tab, you can easily delete scheduled interviews in Greenhouse before rejecting a candidate. Just find the candidate's profile and click Update next to the scheduled interview.
Pressing the Delete from Google button will remove the interview from your calendar, so you can easily Reject the candidate after you remove all of them.