At the end of November we reported that the calendar iCloud was being invaded by spammers and that, although there are ways to work around the problem, there was no simple way to get rid of these invitations permanently. Apple quickly learned everything and said that she was actively working to resolve the problem (identifying and blocking suspicious senders and spam in the invitations being sent).
Today, showing that she was really working to resolve everything, a new feature for the Calendar was released: the possibility to easily report an invitation as unwanted / spam.
For now the feature is only available on the iCloud.com web app, but taking into account that this week the new versions of the operating systems (iOS 10.2 and macOS 10.12.2) have everything to be released, able to see the feature already included in the Calendar app.
The button to identify as unwanted / spam appears only on calendar invitations from unknown senders if any of your circle sends you an invitation, the option doesn't even show up; when you receive it from a stranger and mark it as unwanted, an alert appears for you to confirm the action, warning that the event will be deleted from your calendar.
As we said, the feature is not yet available on iOS and macOS systems, but it should arrive soon; in the meantime, if you have one or more of these invitations in your calendar, you can visit iCloud.com and mark it as unwanted because obviously everything will be synchronized and the event will be deleted from your device (s) ).