function jnj_acuvue_tweak_cas_validate($form, $form_state) {
  if ($form_state['values']['name']) {
    // verify if is e-mail
    if (strpos($form_state['values']['name'], '@') !== FALSE) {
      $type_name = "mail";
    }
    else {
      $type_name = "name";
    }
    // call user_load function
    $user_load = user_load(array($type_name => $form_state['values']['name']));
    if ($user_load) {
      if (in_array('CAS user', $user_load->roles)) {
        form_set_error('name', t('CAS users can not change password.'));
      }
    }
  }
}

function jnj_acuvue_tweak_form_alter(&$form, $form_state, $form_id) {
  global $user;
  global $language;

  // Forbid CAS user changing password
  if ($form_id == 'user_pass' || $form_id == 'user_profile_form') {
    $form['#validate'][] = 'jnj_acuvue_tweak_cas_validate';
  }