Outlook recently added support for Gmail in its Mac desktop client. I added it and was immediately hit with a calendar notification that would not leave me alone. I would dismiss it and it would come back 5 seconds later. I would snooze it and it would come back 5 seconds later. It was really frustrating. All of the information online that I could find dealt with resetting profiles, repairing calendars, clearing caches. None of this worked.
Since the item was on my calendar and I owned the item (a yearly reminder that Sept. 19 is "Talk Like a Pirate Day"), I simply deleted the entry from the calendar (inside Outlook) and the reminder immediately vanished.
Hopefully this helps because you've tried everything else or the other options seem way too drastic (yeah, I'm not telling my IT department that I nuked my Exchange profile) and you were tearing your hair out trying to find a solve.