Unable to Delete Fastic App events from calendar

So, you've followed the steps here?

Delete spam calendars and events on iPhone - Apple Support

Are you syncing any other Calendars, such as those with an Outlook, Hotmail, or Google account? If there is no unsubscribe option, then unsync your other calendars and most likely the problem will be solved.

Have you gone to Settings>Calendar>Accounts and made sure that there isn't a rogue account there?

To remove unwanted calendar events from your calendar, follow these steps: 1) Remove calendar subscriptions by checking your iPhone's settings, 2) Check for calendar subscriptions in the Calendar app, 3) Check iCloud Calendar settings, 4) Delete events from the calendar, 5) Check the Fastic app settings, 6) Remove calendar entries using a computer, 7) Restart your iPhone, 8) Contact Fastic Support for assistance, and 9) Reset calendar Sync (Last Resort) to reset settings without erasing data. These steps should help you resolve any issues with your calendar app.