Hi I am a UK Powerseller and it's not very often we get unpaid items, generally I am sorry to say most people who have tried to rip me off in the past are from the USA or Canada. To combat this I added a small sentence to our payment policy which states that an ebay member with a feedback of less than 10 must email us before bidding & if they don't we reserve the right to cancel their bids, which is your right as a seller if they do not follow your policies. We make a concious effort to contact all foreign bidders and if we get no response we cancel the bid it's that simple. This seems to have worked for us - touch wood - if you do this you must stick to it, you will find that scammers will not email you or respond to your messages.
Oh & before I forget to mention it! Once you cancel the bid remember to block that bidder from bidding within your ebay settings then email them a polite notice stating you have cancelled their bid due to lack of response and if they are a genuine bidder they should contact you so that you can re-instate their bidding rights on your items.
As previously stated the only way to stop these bidders is to block the bidders from countries you do not wish to trade with using the settings within ebay. If you select this preference your items will not be available to the bidders in those countries. You will find that genuine bidders will contact you to ask if you will trade with them.
The other thing you could try is a different payment method for those buyers which offers you protection against fraud better than paypal does,you would have to do a lot of research but it would be worth it in the end for your own peace of mind.
Unfortunately your items are at high risk of scammers but just remember not everyone is a scammer !! We have great International trade which is increasing all the time. I wish you luck. :D