If you use iCloud Calendars, you've likely seen a strange calendar event that you didn't create. I've seen them too. These are spam invitations and should be ignored at any cost. Unfortunately, because of how iCloud invites work, this isn't easy. Apple has told iMore that they are working on a fix. Until then, a good workaround is moving the spam event to a new calendar. Then, delete this new calendar. Doing this will not notify the spammers if your account is real. Credit to MacRumors for that tip.
"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."