Hide products that are out of stock

1. In you Dashboard, place your cursor on WooCommerce and click on Settings
2. Click on the Products tab
3. Click on Inventory (under the tabs)
4. Uncheck Hide out of stock items from the catalog
5. Click on Save changes

Source : http://www.kriesi.at/support/topic/hide-out-of-stock-products-in-woocommerce/