To mark an item as "out of stock" or "inactive", edit the product via "Products > View/Edit Products" in the admin.
you have selected a product to edit, you will see a field named
"Active". Uncheck the checkbox next to this field to mark the item as
"inactive". When an item is inactive, the item will not display on the
site. Customers will not be able to purchase or view the product. The
product is not deleted however and can be put back on your site simply
by editing the product again and checking the "Active" checkbox.