Yes, you can block visitors per their IP address. Countries will have a specific IP address range, and you can use that information to block all or some of their IP's.
For a current list of IP addresses by country, please visit http://www.countryipblocks.net/
On the resulting page, click the ".htaccess deny" link for the desired country. This is the exact code you should paste in your .htaccess file.
Usually the .htaccess in your public_html folder is best, as it can protect all your addon domains and subdomains.
