Unwanted Calendar Event Spam - NO "subscribe" or "delete" so cant get ride of it

I am also having exactly the same problem, and there doesn't seem to be ‘a fix’ for it that I have been able to find. What I have done so far is to remove it from my calendar by doing the following

1. open Calendar app
2. Click on the little red envelope symbol at the top left of the calendar, it is the second red symbol.
3. Click on the invitation that opens - for me the only one there was the spam invite. I then highlighted it and chose ‘cut’.
4. This appears to have taken the event from my calendar, but I can still see it if I click on the ‘red plus’ symbol - the 4th symbol at top left of the calendar. I am still worried that the spammers may have access to my iPad, but the event has disappeared from my calendar.
5. Good luck with sorting your calendar. I hope there will be an easy and permanent solution sorted by Apple.