How do I undo an import into macOS calendar?
It sounds like your calendar event was generated from Google Calendar as an event which 'repeats itself' a number of times. If that is the case, and if you have imported this event in a calendar which you have linked to a Google account, you should be able to open it from Google Calendar and remove the event from there, including all the recurrencies of the event. When you do so, you can choose not to notify others of these deletions.