Really, your buyer in Serbia should've noticed that there's no postage method or price to Serbia showing on your listings so should not have bid, or at least contacted you to enquire about it first.
Cancel the orders, as advised, and then set your Buyer Requirements with this page, tick the box that says 'Block buyers whose primary delivery address is in a location I don't post to, then the box that says 'Apply above settings to active and future listings', then click on the tiny Submit link, bottom left of this page:
https://offer.ebay.co.uk/ws/eBayISAPI.dll?BuyerBlockPreferences
Once you've done that, if there's no postage price and method to their country, it blocks bidders.
This will only be applied to any fixed price Buy It Now listings you have up and running, it will not be applied to Auction style listings that are currently live, only to future ones.