AJAX event and disable select form element
try:

$form['select']['#attributes']['disabled'] = 'disabled';


Ok, a detailed explanation can be found on this article https://www.drupal.org/node/357328

But pretty much resumes to: 


  we cannot simple set the $element['#disabled'] attribute of the
  element. The reason is this attribute is transformed into
  $element['#attributes']['disabled'] by
  _form_builder_handle_input_element executes()...