How to disable one sort/direction combination from a combined Better Exposed Filters field?
I can't find a way to do it through Views or Better Exposed Filters settings, but I have found a way through hook_form_alter() in the theme template.php file:

Something like this, specific array keys are based on using the BEF combine option and may vary:

function YOURTHEME_form_alter(&amp;$form, $form_state, $form_id){
  switch ($form_id)  
  case 'views_exposed_form':
    if($form['#id'] == 'views-exposed-form-VIEWID-DISPLAYID'){
      if( isset($form['sort_bef_combine']['#options']['field_FIELDNAME ASC']) ){
        $form['sort_bef_combine']['#hidden_options']['field_FIELDNAME ASC'] = $form['sort_bef_combine']['#options']['field_FIELDNAME ASC'];
        unset($form['sort_bef_combine']['#options']['field_FIELDNAME ASC']);
      }
    } 
  break;
  }
}