Whether a product can still be ordered in your shop at stock 0 is controlled by Shopify, not Pickware. What matters is the Continue selling when out of stock setting on the product in Shopify. If it is active, the product stays orderable, even if no stock is available in Pickware anymore.
How to prevent orders without stock
Open the product in Shopify and deactivate the Continue selling when out of stock option. After that, the product can no longer be ordered as soon as no stock is available.
Note: Shopify does not evaluate the Clearance sale switch in Pickware Admin. It is not transferred to Shopify and has no effect there.