In most cases it's preferable to set products to Inactive rather than deleting them. Deleted products will be created as New products if they happen to show up in a future order import.
Once a product has been changed to Inactive it is possible to delete it from the Inactive products list. Select the products you wish to deleted from the inactive list and you'll find the delete option in the edit drop-down in the Inactive list view.
Please note that when a product is deleted it is removed from any orders it is present in. You'll see a notification of this if you open any of those orders.