If a would-be user posts a request to become a user, it is easy enough to approve them. But I can't find a way to disapprove
! That is, to reject the request but at the same time to send a helpful message saying why the request is not approved.
Background: I have an app in which I have already registered several hundred users. One or two of them ignore the email that email@example.com has already helpfully sent them to tell them how to login, and instead try to create a new account.