You have to reply. Reply politely and explain the transaction is null and void because you put in your description no international sales. Tell him you are initiating a mutual transaction cancellation and for him to please accept it. Just be polite but specific.
There is always a risk of a negative strike, but he probably won't. Even if you do, there is a chance you can get eBay to remove it. Just follow the eBay rules to the T, that starts with communicating. If he stops replying and/or refuses to accept the mutual cancellation, you will have to wait the 7 days. If that happens, wait and file again.
BTW, I just remembered. IIRC with unpaid item dispute there is an option for "Buyer from a country I don't ship to".
Good luck.