@cagatay the fastest way to do this would be to modify the auto generated sitemap.xml file so that it does not index users. It might be paint to do the same thing with users in terms of guest permissions.
Let me have a look.
Edit - you can do this with permissions. Go to /admin/manage/privileges then look in the left where it says guests - remove the tick from the view users permission for guests then click save
You can test this out using an incognito or non logged in session. Attempting to view users should then ask you to login.
