I'm not sure if theres going to be an easy way to do that. You've probably gotta do a two step method.
First find the country of your spammers using just about any simple WHOIS will return that.
Then you can get an entire country IP range
http://www.countryipblocks.net/
But you should probably be warned that spammers IP changes just like anyone elses, so you might want to be careful blocking entire countries by ip address