Open the Calendar app
Tap the spam event
Tap Delete Event
If prompted, choose Delete Event again
Open Settings
Tap Calendar
Tap Accounts
Check for suspicious subscribed calendars
Tap Subscribed Calendars
Select the spam calendar
Tap Delete Account or Unsubscribe
Open Settings
Tap Calendar
Tap Accounts
Remove any unknown calendar accounts
Open Calendar app
Tap Calendars at the bottom
Uncheck the spam calendar if it still appears
Open Settings
Tap Calendar
Tap Siri & Search
Turn off event suggestions if needed
Open Settings
Tap Calendar
Tap Sync
Change sync range if spam keeps reappearing
Restart the iPhone
Update iOS to the latest version
If spam continues, remove the calendar source from Mail, Contacts, or subscribed links