Magento – Item out of stock in admin – in stock on frontend

Check your system configuration for values set at different store/website levels that might override your global settings.

Check your individual products for configuration that might override your system configuration levels.

Try setting a quantity to 1, purchasing a product, and checking is things are marked out of stock. (i.e. the products you see may have been explicitly set to be in stock with a quantity of 0 before you made your change. If this is the case and the system is otherwise behaving, I'd try setting those products to be out of stock manually)

You have to change the status for each product.

The good thing is that it has been done before:

How to set a product to be in/out of stock programatically

You have to loop through your products and apply the snippet in the above link.