Spam iCloud Calendar Invitations
Regarding how to prevent this in the future (question #3):


Log in to iCloud Calendar on a computer web browser
Authenticate if needed, then select the Calendar view
Select the Settings Gear icon ⚙ in the bottom left corner then hit Preferences...





Select Advanced


Choose the option to receive calendar invitations by email instead of allowing the server to insert them without review into your calendar:




Select Email to [youremail] instead of In-app notifications


You now get to decide and delete or process calendar spam like regular email spam, allowing you to filter out these spam calendar invitations systematically.