To exclude bidders bidding from countries that you don't post to you have to set up your buyer requirements to reflect this.
See here
http://pages.ebay.co.uk/help/sell/buyer-requirements.html
Now that the person has won you may as well find out the postage rate for their part of the world (google royal mail postage prices) and then adjust the invoice to add the extra.
You don't have to honour the sale if you don't wish and can cancel the sale in the Resolution Centre citing 'buyer from country I don't post to' but it's a bit of a faff and you're just as well to adjust the invoice and post the item as usual on this occasion.
Green is the new pink.......