->add('store_orders', 'sonata_type_collection', array(
'btn_add' => false
))When you add a collection to Sonata admin forms, by default an "Add New" button is displayed, to prevent the "Add New" Button or "+" from displaying, set the add_btn key to FALSE in the array, which is the third parameter in the add function.
'type_options' => array('delete' => false)