Do you mean you can still purchase through POS even though you marked the product as 0 inventory in admin? If so note the following:
If you're using Shopify POS, you can continue selling products when inventory reaches zero and below. This is because the Shopify POS app assumes you have the inventory on hand if you are selling in person. https://help.shopify.com/en/manual/sell-in-person/shopify-pos/inventory-management/setup-inventory-t...