I upgraded to Nextcloud 10.0.1 today (from 9.0.5) and for some reason some old test accounts I had previously deleted showed up under my subscribed calendars. When I try to delete them nothing happens, and when I try to delete them from the calendar app on my Mac it shows a “403” error. Does anybody know how to delete these subscribed calendars via command line?
There is no terminal command to delete the calendar. If you get an 403 error, there is perhaps something not right in your setup (check also your logfiles for more details).
Do you still see these test accounts in the user list?
No, I had deleted them a long time ago, for some reason their calendars re-appeared when I upgraded form 9.0.5 to 10.0.2
No ideas on how to delete the ghosts calendars?
You could certainly delete the entries in the database manually. The proper way would be to have some repair procedure, to keep track of such issue, the best is to put this on the bugtracker at https://github.com/nextcloud/calendar/issues
The calendar repo is the wrong place, please report this in https://github.com/nextcloud/server/issues 
thx