Apple recently released a statement, saying it is working to address the issue by identifying and blocking suspicious invite senders. Apple acknowledged the issue in a brief statement to iMore's Rene Ritchie, who shared the response via Twitter. It reads: "We are sorry that some of our users are receiving spam calendar invitations. We are actively working to address this issue by identifying and blocking suspicious senders and spam in the invites being sent.”
Since iCloud calendar invites are sent to an iCloud calendar automatically by default, I’m not sure how Apple will deal with the issue. But at least the company is working on it.