Its possible; but depends on what countries.
You could potentially block countries based on the RIR's that the IANA allocates IP address ranges too. Take a look at http://en.wikipedia.org/wiki/Regional_Internet_Registry
which explains the different RIR's that exist and the countries they compromise.
The problem you may have though is that IP's ranges are allocated per region (ie. Africa) and not per country (ie. Zimbabwe) - thus for example if you want to block say the USA, but not Canada, this won't be possible.
Technically blocking a region is easily achievable through iptables (once you've figured out what the IP address range is that you want to block).