There should not be the plus nor duplicate icons in the Users screen/form as this way of adding users does not work end ends up with error any time used.
There are situations where there’s need to add user without access to the application. Prohibiting add button would result in need to add a user with workflow Add User and then to remove the access from Users screen.
I agree with removing the copy button, but I suggest to keep add button. Instead make fields UserName and UserEmail read only.
We can actually not add manually anymore even for Windows Integrated Authentication as OrigamUser record is needed. (We work around it with a database trigger).
It would be solved automatically if Change BusinessPartner to OrigamUser in model would be solved.