You can use the
robots.txt file to disallow the indexing. Try using this directive in your
The cons of this approach is that not all search engines crawlers observe the wildcard syntax. I think this will do the trick with the major ones (Google, Bing and Yahoo).
I found this and this useful articles about robots.
Another way that you could try is in every menu item, that calls the com_users component, in the metadata tab, set the robots option to No Index.