ajax_test.php

/**
 * Form callback
 */
function ajax_disable_test_form($form, &$form_state) {
  $form['select'] = array(
    '#type' => 'select',
    '#required' => FALSE,
    '#title' => t('Change me!'),
    '#empty_option' => t('- Select -'),
    '#options' => drupal_map_assoc(range(1, 3)),
    '#prefix' => '<div id="select_box_wrapper">',
    '#suffix' => '</div>',
  );
  $form['change'] = array(
    '#type' => 'submit',
    '#value' => t('Change!'),
    '#ajax' => array(
      'callback' => 'ajax_disable_test_form_submit_ajax',
      'wrapper' => 'select_box_wrapper',
    ),
  );
  return $form;
}

/**
 * Ajax callback
 */
function ajax_disable_test_form_submit_ajax($form, $form_state) {
  $form['select']['#title'] = t('Changed!'); // It's work!
  $form['select']['#disabled'] = TRUE; // It's not work!
  return $form['select'];
}

https://www.drupal.hu/forum/ajax-esemenyre-legordulo-kivalaszto-letiltasa/22861
http://drupal.stackexchange.com/questions/178599/ajax-event-and-disable-select-form-element