Over the last few weeks many people have been reporting unwanted Calendar invites, usually looking like junk email with a sales pitch and links. It is difficult to get rid of these invites without notifying the sender that you either declined or accepted it. But there is a method to quietly delete the invite. A better solution is to switch to email notifications of Calendar invites until Apple fixes the problem.
Update: You now have a “Report as Junk” button on Calendar invites if you look at the Calendar in iCloud.com. So if you get one of these, the best way to handle it now is to log on to iCloud.com in the browser, view the invite in the Calendar app there, and hit that link. It deletes it without notification and also tells Apple about it so they can track and perhaps block that IP address or block. I expect this same button/link to appear in the Calendar app in iOS and Mac at some point in the future.