How do I block unwanted calendar invitations?

The first thing to be aware of is that you should avoid selecting ‘Decline’, this will only alert the sender that the email address is valid and encourage them to send more.

To remove the the invitation without selecting ‘Decline’, create a new calendar and name it something like spam. Open the invitation and select the new calendar to move it to and then delete the new calendar.

To prevent further invitations arriving in your calendar, you might want to consider changing your calendar preferences at iCloud.com > Calendar > (Gear Icon) > Preferences > Advanced so that you receive invitations as email rather than in-app notifications. You can then easily delete spam invitations without alerting the sender they have hit on a valid address or even set up rules to del with persistent spam sources.