How are these Chinese spam iCloud Calendar/Reminders invitations sent to me? Recently, I’ve received a couple of spam event invitations in Calendar on my Mac. The most recent had only Chinese text, and recurred at 5:46am every day.

I am able to decline these by clicking on the invitations button in the toolbar (that only appears when an invitation comes in). However, ideally I’d like to not receive these spam invitations at all.

As I’ve never received a genuine invitation through Calendar, I thought I might be able to turn off the invitation feature. However, I can’t see any controls for this in Calendar preferences or the Internet Account system preferences.

Through what mechanism do these invitation arrive? If I understood what mechanism it was, I might be able to turn it off. I have two Calendar accounts enabled: iCloud, and Facebook. I can’t see anything in my e-mail, so I presume they’re not regular e-mail spam.

Since first writing this question, I’ve had a similar invitation come through in Reminders:
It's simply that anyone can send an email to you. If spam rules don't block it, Apple software (with default settings) processes the invitation before you can take any action.

To prevent this automatic processing, try this. 

Log in to iCloud on the web, open your calendar and go to ⚙ > Preferences (in the lower left corner). Under Advanced tab you’ll find an option to receive calendar invitations by e-mail instead of directly to your calendar.