If you don't want to sell overseas you will need to set your buyer requirements to block countries you don't want to post to. If you don't there is every chance overseas buyers will continue to bid on your items. If you haven't listed overseas postage they should really have emailed you before they bid to request revised P&P cost.
Most people on AC will advise that it's simple to post overseas, which it is, just a bit more expensive. But that's if it all goes smoothly. If it all goes pearshaped it can work out quite expensive.
If you still would rather not send then contact the buyer and explain and ask if they will accept a cancellation request. If they are unwilling, you could try Ebay CS and see if they will cancel as an Unwelcome Bid.
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