Voici mon exemple (en utilisant #after_build):
$form['legal']['legal_accept']['#type'] = 'checkboxes';
$form['legal']['legal_accept']['#options'] = $options;
$form['legal']['legal_accept']['#after_build'][] = '_process_checkboxes';
Plus le rappel de fonction suivant:
function _process_checkboxes($element) {
  foreach (element_children($element) as $key) {
    if ($key == 0) { // value of your checkbox, 0, 1, etc.
      $element[$key]['#attributes'] = array('disabled' => 'disabled');
      // $element[$key]['#theme'] = 'hidden'; // hide completely
    }
  }
  return $element;
}
Testé sur Drupal 6, mais il devrait également fonctionner pour Drupal 7.
Drupal 6
Vous pouvez utiliser la fonction suivante ( source ):
/*
 * Change options for individual checkbox or radio field in the form
 * You can use this function using form_alter hook.
 * i.e. _set_checkbox_option('field_tier_level', 'associate', array('#disabled' => 'disabled'), $form);
 *
 * @param $field_name (string)
 *    Name of the field in the form
 * @param $checkbox_name (string)
 *    Name of checkbox to change options (if it's null, set to all)
 * @param $options (array)
 *    Custom options to set
 * @param $form (array)
 *    Form to change
 *
 * @author kenorb at gmail.com
 */
function _set_checkbox_option($field_name, $checkbox_name = NULL, $options, &$form) {
    if (isset($form[$field_name]) && is_array($form[$field_name])) {
        foreach ($form[$field_name] as $key => $value) {
            if (isset($form[$field_name][$key]['#type'])) {
                $curr_arr = &$form[$field_name][$key]; // set array as current
                $type = $form[$field_name][$key]['#type'];
                break;
            }
        }
        if (isset($curr_arr) && is_array($curr_arr['#default_value'])) {
            switch ($type) { // changed type from plural to singular
                case 'radios':
                    $type = 'radio';
                    break;
                case 'checkboxes':
                    $type = 'checkbox';
                    break;
            }
            foreach ($curr_arr['#default_value'] as $key => $value) {
                foreach($curr_arr as $old_key => $old_value) { // copy existing options for to current option
                    $new_options[$old_key] = $old_value;
                }
                $new_options['#type'] = $type;  // set type
                $new_options['#title'] = $value;  // set correct title of option
                $curr_arr[$key] = $new_options; // set new options
                if (empty($checkbox_name) || strcasecmp($checkbox_name, $value) == 0) { // check name or set for 
                    foreach($options as $new_key => $new_value) {
                        $curr_arr[$key][$new_key] = $value;
                    }
                }
            }
            unset($curr_arr['#options']); // delete old options settings
        } else {
            return NULL;
        }
    } else {
        return NULL;
    }
}
/*
 * Disable selected field in the form(whatever if it's textfield, checkbox or radio)
 * You can use this function using form_alter hook.
 * i.e. _disable_field('title', $form);
 *
 * @param $field_name (string)
 *    Name of the field in the form
 * @param $form (array)
 *    Form to change
 *
 * @author kenorb at gmail.com
 */
function _disable_field($field_name, &$form) {
    $keyname = '#disabled';
    if (!isset($form[$field_name])) { // case: if field doesn't exists, put keyname in the main array
        $form[$keyname] = TRUE;
    } else if (!isset($form[$field_name]['#type']) && is_array($form[$field_name])) { // case: if type not exist, find type from inside of array
        foreach ($form[$field_name] as $key => $value) {
            if (isset($form[$field_name][$key]['#type'])) {
                $curr_arr = &$form[$field_name][$key]; // set array as current
                break;
            }
        }
    } else {
        $curr_arr = &$form[$field_name]; // set field array as current
    }
    // set the value
    if (isset($curr_arr['#type'])) {
        switch ($curr_arr['#type']) {
            case 'textfield':
            default:
                $curr_arr[$keyname] = TRUE;
        }
    }
}