Got a scam alert in calendar can’t delete

iPhone Calendar app > remember the color shown by the calendar spam > select the Calendars button at the bottom > scroll down to the Subscribed calendars > find the calendar with the matching color in the subscribed section > select the info button to the right of the same-color entry > scroll down and select delete.

iPhone/iPad with iOS13 and earlier: Settings > Passwords and Accounts

iPhone/iPad with iOS14: Settings > Calendar > Accounts

Look for a calendar subscription you don't recognize. Tap the unknown calendar and choose to delete it.