How do I prevent automatic fulfillment of pre-order products and ensure they don't go through to fulfillment?

If you have automatic fulfillment enabled, it is not possible for Pre-order Now to prevent an order containing pre-order items from reaching your shipping and fulfillment app. 

You may need to disable automatic fulfillment of orders. We recommend reviewing Shopify's documentation on this topic to learn more about your options:

https://help.shopify.com/manual/orders/fulfill-orders