1) Untested but should work: in include/menubar.inc.php line 228 find

    $block->data['calendar'] =
      array(
        'URL' =>
          make_index_url(
            array(
              'chronology_field' => ($conf['calendar_datefield']=='date_available'
                                      ? 'posted' : 'created'),
               'chronology_style'=> 'monthly',
               'chronology_view' => 'calendar'
            )
          ),
        'TITLE' => l10n('display each day with photos, month per month'),
        'NAME' => l10n('Calendar'),
        'REL'=> 'rel="nofollow"'
      );

and comment that entire assignment out, i.e. place /* before the block and */ after the block. The change will get overwritten by an update so you'll have to redo it.

2) The _data directory and its subdirectories also need to be writable for the web server user. Some extensions write to the local directory. And of course the entire tree for upgrades. The plugins/ directory IMHO needs to be writable only for updates.

Hi :-)

personal plugin

add_event_handler('blockmanager_apply' , 'remove_item_menu', EVENT_HANDLER_PRIORITY_NEUTRAL+10);
function remove_item_menu($menu_ref_arr){
  if (($block = $menu_ref_arr[0]->get_block('mbSpecials')) != null){
    unset($block->data['calendar']);
  }
}