How to get rid of a spam event in Calendar

I have a spam event in my calendar app. There's no option to unsubscribe from a Calendar at the bottom of the screen. Nor is there an unrecognised account under settings.

It says it is an Outlook calendar event, however it does not show on Outlook.

None of the steps here Delete spam calendars and events on iPhone - Apple Support apply.

The spam event appears again after 1-2 seconds after I decline - there is no other option to delete or to report it as junk.

Help!