Security issue...iCal Spam event invite.
iPhone 6s, iOS 9.0.1, null
Get help using iCloud Contacts, Calendars, or Reminders - Apple Support
According to this article : If you get a spam iCloud Calendar request
If you get a calendar request that you think might be spam or junk, let us know. Sign in to iCloud.com with your Apple ID and go to the Calendar app. Open the event that you wish to mark as Junk, click Report Junk, then click OK. It will automatically delete the event from your Calendar on all your devices signed in with the same Apple ID.
You can change the password for Apple ID & even the questions & answers .
Can use : Two-factor authentication for Apple ID - Apple Support
: Availability of two-factor authentication for Apple ID - Apple Support
I've seen it mentioned (with same picture) on other parts of this forum (iCloud area). My suggestion would be to follow the steps here -- If you think your Apple ID has been compromised - Apple Support
Consider changing your password to something stronger or even using a third-party password protector, e.g. 1 Password.
Also, consider enabling two-factor authentication or two-step verification for your Apple account.
Security issue...iCal Spam event invite.