Hi,
If you set the quantity of a product to 0, the add to cart button and the quantity box will be removed.
If you just wan to display the quantity box and keep the add to cart button for all the product, you can change the configuration option "Display the quantity field on the product page" under the tab display.
But if you want to do it only for some products, you will have to change the code of the view. You can go to Display->Views and edit the file show of the view product. There, you can use the code
$this->params->set('show_quantity_field',0);
to hide it, and the code
$this->params->set('show_quantity_field',1);
to show it.