I created a product with 2 Option groups - Color and Size - utilizing drop-down menus.  In the variant inventory, I left unchecked the combinations that are not available, e.g. large and green.   When viewing the product, the drop-down menus allow you to select the opton1 large and option 2 green.  The item goes into the cart, but displays an unavailable message, that is not that obvious, considering the unavailable combination is displayed in the cart.      

 How do you set the variants so the unavailable combinations from the 2 option groups cannot be selected?