Talk to your boss.
Maybe your boss asked to get cc'd. Maybe your boss also disagrees with your decisions too, and you should look into doing things differently. Maybe your boss is wondering why you're not taking care of things (and can offer advice).
But mostly, if your boss hasn't already had a talk with your subordinate to knock it off, he/she has tacitly approved of the cc-ing and is undermining you.
Having your boss politely decline this "helpful information" is probably the least jerky way to handle things. It provides you with the support you need, while hopefully letting your subordinate know that such things are inappropriate (in this case) and getting them back to working with you.