How can I prevent spam events going into my iPhone calendar?

Do you have an Outlook or Hotmail account?

Recently spam events have come from those account and you may need to shut off syncing those accounts with the Calendar. There has been much discussion on the Microsoft Support sites for users receiving Calendar Spam through the accounts across other devices as well.

If that is not the case for you, then Apple has this Support Article for deleting the Spam and the Spam Calendar that may have been inadvertently subscribed to.

Delete spam calendars and events on iPhone - Apple Support