How can I hide exposed filter control if the filter is not applicable to existing views results?
You have to create a small module or add a function into template.php where you use a one of the hook alter functions. You test your view results for a given set, and if you at that result don't need the filter you unset it. Probably the best hook is: hook_views_pre_render.

"https://api.drupal.org/api/views/views.api.php/group/views_hooks/7"

Check out the comments on the hook_views_pre_render function to get an idea of how to use it. Hope this gets you on your feet.