New Discourse user here, playing with hosted instance since this morning, and now fiddling with authentication/authorization.
Hitting the exact same issue: a colleague of mine signed-up locally (note: I’ve also enabled a few oauth2 providers), then logged-out, then as admin I went to delete him (delete only) → no can do 
(There was an error deleting that user. Make sure all posts are deleted before trying to delete the user.)
Maybe relevant:
- he has the discobot greetings PM in his inbox, deleting it did not make any difference
- I got him to create a topic and then log back out, still can’t delete him
- his email domain matches an auto-join primary group I defined (can’t remove him from the group either in fact, which I’m not sure is a feature or an anomaly given that his domain does match)
Let me know if any further info could help, I’ve tried to share as much relevant info from what I learned in just a day (started trial this morning).
Thanks,
Nicolas
Note: noticed other traces of such behaviour in this much more recent post
But that issue is closed, hence why I reached out here