Deleting a user
If you choose to delete a user from the User Details page, click the delete button.
You should then see a dialogue warning you that you are going to delete the user. At this point you can either cancel, or continue with the delete.
Confirmation of Delete
If the deletion of a user has been successful, a "user deleted" confirmation will appear.
Keep in mind that this action does not delete the user from your database, unless you have customized this process using a Custom User Store. Instead, it simply flags the user as deleted.
Note: Your OpenIddict solution would need to make use of this flag for it to have any effect. Otherwise the user could still log in after deleting.