Deleted event in calendar keeps reappearing

I have an iPhone on iOS 11 and a MAC on El Capitan.

I have a recurring event and simply wanted to delete one occurrence.

I first tried this from my phone. I opened the event clicked 'Delete Event' and answered the prompt to 'Delete this occurrence only'.

The event disappeared momentarily then returned again.

I tried the same thing on the Mac and again it returned.

I tried logging into iCloud to delete it from there but yet again it returned.

I've tried going into settings on the phone and on the Mac.

I know the name of the calendar within the respective apps but I can't find any way to identify where/what/who this calendar is synching with - if any! I set this event up almost 2 years ago and can't remember how I did it.

This is infuriating because it was supposed to be a quick, simple thing to do just to avoid getting an unnecessary alert. I've accomplished that simply by removing the alert in the event itself - that's not the real issue. The issue is why can't I delete an event!... and how do I find the originating source.