To delete a product and a product-item variety, you first need to activate the corresponding rights in the user group. When these rights have been activated, a "delete" button will appear at the bottom of the product page (opposite the "Save" button). When you delete a product, the varieties of that product are also deleted.
Each product-item variety has a delete button in the upper right corner (except for the first one, which is the main product). Upon clicking, only the selected item variety is deleted.
When you delete a product-item variety, it is not deleted from the order details, rather it becomes inactive.