This may help. Look in the calendar event and note the sender. Then go into email and search for that sender. When it happened to me, I didn't see the email from the sender just by scrolling down through email folders. I had to use the search function. If you find the email, then delete it and that will remove from your calendar. Additionally, note the email address it was sent to, or which calendar the spam is in, if you have more than one email address setup I your mail app. That will help narrow your search space.