There's been a proliferation of spammy calendar invites lately, mainly revolving around sales of "brand name" products such as Ray-Ban sunglasses. I've gotten a few of these, and they're really annoying.
There's a workaround, but Apple provided iMore with an official statement addressing the issue:
"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."