Products that are out of stock in the eCommerce Shop can still be purchased.
Admin: eCommerce Admin
Page: Company Settings > Carts
Permission: Setup Carts
When configuring a Cart in the eCommerce Admin:
Add the stock settings as Show out of Stock and prevent users from ordering.
Enable the IsOutofStock toggle.
See: Adding Stock Settings for Shopping Carts in eCommerce Admin to complete this process.