How do I eliminate a country I won't ship too?
I read that many times, item don't get delivered in some countries
How do I block buyers from these countries?
From the Members Area > Selling > Seller Tools > Postage Setup page, you can setup your Postage Zones, which include all of the countries you ship to. There are two default Postage Zones, one for domestic, and one for international. However, if you only want to ship to certain countries. You can replace the "special" country "Everywhere Else" which represents all other countries you do not explicitly set, and instead just add which countries you do want to ship to. There's no "won't ship to" option, but just leave out whatever countries you do not want to ship to.
You will only need to do this once, since each listing is tied back to the Postage Zones you have set per the above.