I got a fairly explicit one of these in my calendar the other day, unfortunately it's a calendar I share with my wife, so it appeared on her phone too. That was a fun conversation.
Neither of us could delete the event, either via google calendar or ical. Nor could I find the original email I assume it came from. In the end I just deleted the whole shared calendar.
I had a similar experience, eventually I found the message in my spam folder. It's ridiculous that messages marked as spam show up on the calendar, but now we know.
One of the problems with the suggested solution in the article is that it doesn't apply to other people's view of your calendars - so my partner had an event from my calendar clogging her view but I couldn't see it to remove it!
Weirdly enough, I had an easy "Mark as spam" button on iCal as well as in Google Calendar. I never needed to turn off the auto-event feature because it went away when I marked one as spam.
I wonder if you got a different type of spam than I did.
The awesome part is the steps you take to make it not show up on your own calendar don't actually make it go away. It's still there and will appear for people you share the calendar with.
Neither of us could delete the event, either via google calendar or ical. Nor could I find the original email I assume it came from. In the end I just deleted the whole shared calendar.