How do I purge ancient invisible calendar events?

There is a meeting still on my X1 that was recurring and on my calendar three years ago, and not since then. This meeting doesn't show in my Apple or Outlook calendars on my Mac, nor in Calendar on my iPhone. I can't find any way of deleting it for that reason. 

I presume it was on my (non-X1) Venu for that whole time, but I never noticed it.

How can I get these to vanish?