I want this concept to list only user names (preferably without the User: prefix) and definitely not list subpages within the User namespace. (It could be any namespace: Template: News: etc.)

Having studied 
 (i) http://semantic-mediawiki.org/wiki/Help:Concepts
and
 (ii) http://semantic-mediawiki.org/wiki/Help:Selecting_pages
I try:  

{{ #concept: [[User:+]] }}

However this concept also lists subpages within the User namespace.

So I tried these variations on the same theme:
[[User:+/]]
[[User:+!/]]
[[User:!+/]]
[[User:!/]]
[[User:+]] [[!/]]
[[User:+]] [[User:!+/]]
[[User:+]] [[User:!/]]
[[User:]]
[[User:~+/]]

Still no joy!

How might I modify this concept query to exclude any subpages in the target namespace?
I don't want to presume that all user pages:
 (a.) a have semantic data
or
 (b.) belong to a specific category

What page(s) can I read to learn more about this subject?