Rogue calendar events

In addition to the above > Delete spam calendars and events on iPhone - Apple Support

If you use the Calendar path to find and delete the subscribed calendar rather than the Settings path, you can use the calendar color shown for the events to help identify the problematic subscribed calendar in the listing; the calendar colors will match.

Settings>Calendar>Accounts... delete the rogue calendar account.

When done, restart your iPhone

Regards

Giulio