How do I get the iPhone calendar to stop reminding me about meetings that no longer occur / happened in the past?

Every morning, day and night I receive alerts on my iPhone 6 (latest iOS as of 11/15) from the calendar reminding me about meetings that no longer take place. I have tried deleting them from Outlook and Exchange (the source of the invites), I have repeatedly removed them on the iPhone calendar, and some of them will not let me delete them. This is especially annoying because some of these events occurred over two years ago.

This problem first occurred when I upgraded to iOS 11.

Any advice would be most helpful. Thank you.