@Buscar웃 pointed me in the right direction. I have upvoted that answer.
Disclaimer: I'm no Calendar.app expert. Please don't follow these directions if you're uncertain of how your calendar events are stored and you don't want to accidentally delete something that's unrecoverable.
~/Library/Calendars in Finder and keep it open.
Look at the list of folders that it contains. There will be a folder for each calendar. These folders are named with a random string at the beginning followed by a dot and an abbreviation that represents the type of service used for that calendar. For example:
In my case, I knew that the mystery calendar would be one of the folders that contains ".calendar" in the directory name. Go in to each one of those folders and look at the contents of its Events directory. You will see a list of .ics files. Selecting one of those .ics files will show you a Finder preview of the file, which actually allowed me to see the name of the event that it represented.
So, go to your calendar, and add an event to the mystery calendar. Name the new event something silly that you know is a unique name for an event. I used "Fizzle".
Now open Terminal (or other) and go to the same directory you have open in Finder:
$ cd ~/Library/Calendar
grep to search for the name of the new event.
$ grep -r "Fizzle" ./*
Hopefully you'll get only one result that will show you the full path to the .ics that contains an event named "Fizzle".
Use Finder to locate the top-level directory for that calendar. Delete that directory. You'll also need to delete Calendar's cache files:
- Calendar Cache
- Calendar Cache-shm
- Calendar Cache-wal
Quit and reopen Calendar. The mystery calendar should no longer be available.