The only way to achieve this in Kentico 7 is via customization. The best location is probably in AddProductToShoppingCart method of ShoppingCartItemSelector Control, which serves as add to shopping cart button. Here you can include your custom logic that validates the ProductOptionsParameters before allowing adding to shopping cart. You may of course want to add a similar validation directly to the shopping cart logic.
Kentico 8 comes with this feature out of the box with the new Variants feature. You could save yourself the work on this customization by upgrading your project.