The Buyer Requirements block is based on the bidder's primary eBay address, not on their country of registration. Many bidders who are registered overseas have US shipping addresses. You might want to check with the buyer about this, if there's time to do so before the auction ends.
Depending on which listing program you are using, you can also block non-USA Buyers in a single listing. Besides removing this Buyer, don't forget to revise your listing if there is still time to revise it so someone else doesn't do the same thing.
Look under the more actions tab to the right of your active auction listing for the item, there you should find the cancel bid tab.
However you have put yourself in a precarious position by allowing international bidders to bid if you don't intend to honor those bids.
You need to go to your account settings and change the eBay default from worldwide shipping to only those countries you wish to ship to, mine is set for USA only.
First cancel the bid here:
Then set your buyer requirements in My Ebay, Account, Site preferences to block bidders in countries you do not ship to.
If you set your blocks properly, he can't bid unless he has a US address as his primary shipping address. Anyway, here:
You can cancel his bid using that form.
Sellers can add buyer requirements to their listings to prevent unwanted bidders. You can block buyers who:
Don't have a PayPal account
Have received 2 Unpaid item Strike(s) within 12 month(s)
Have a primary shipping address in countries you don't ship to
Have 4 policy violation report(s) within 6 month(s)
Have a feedback score equal to or lower than -1
See how to access this tool, http://pages.ebay.com/help/sell/buyer-requirements