You can use hook_form_later, find your form name,
below code can help you , form api doc

function yourmodule_form_alter(&$form, &$form_state, $form_id) {
if($form_id=='your_form_id'){
 hide($form['#submit']);
$form['mymessage']= array(
   '#type' => 'markup',
   '#markup' => 'sorry , no one available in room',
   '#weight' => 0,
);
}
}

You can use hook_form_alter. Find your form name and use the code below (form api doc).

function yourmodule_form_alter(&$form, &$form_state, $form_id) {
if($form_id=='your_form_id'){
 hide($form['#submit']);
$form['mymessage']= array(
   '#type' => 'markup',
   '#markup' => 'sorry , no one available in room',
   '#weight' => 0,
);
}
}