I first started receiving spam event invitations in my calendar about a month ago — a vector I didn’t know was utilized — and have been seeing them constantly since. I wasn’t able to figure out a way to prevent them, but Aaron Douglas did. Perhaps the most perplexing aspect of his solution is that it requires signing into iCloud.com; I wasn’t able to find the same controls on MacOS or iOS.
Update: Turns out that this is a more widespread issue than I first thought. Even after performing the steps suggested in the linked article, I received a couple of spam notifications this afternoon, though it might be an issue with the syncing or propagation of the new settings.
One way to deal with these invitations is to simply decline them. However, that will indicate that the email address is in use; I noticed a significant increase in the number of these invitations I received after declining one. Unfortunately, Calendar on MacOS and iOS does not appear to support simply ignoring an invitation.
However, you should know that Fantastical does support ignoring and deleting a calendar invitation without notifying the sender. It’s probably your best bet for now. Hopefully, Apple can fix this by passing calendar invitations through a standard spam filter.