Although it takes more work and effort, we do selective SMTP blocks against countries. Generally we will look at the entire IP range for the provider that is repeatedly spamming and see what their Senderbase score is (http://www.senderbase.org/). If there is not a single Neutral or Good reputation in that entire IP Range (up to a /16) then we block it without hesitation, otherwise we block the smallest IP Range necessary to get rid of the ones with consistent Poor reputations (more often than naught blocking a /24 CIDR Block is sufficient and in some cases will block just an individual IP).
Although we get hit a lot by China (more Brute-Force than Spam), we have customers who rely on Chinese manufacturers and distributors to do their business, preventing us from blocking the entire country wholesale. Not everything in China is unwanted, so it's important not to throw the proverbial baby out with the bathwater which is why we selectively block.
To be honest, the only times we've had to block an entire country was the Ukraine and India (although at this point I'm pretty sure half of the Netherlands are blocked by our Mail Servers), but if you do block an entire country it's important to consider it a temporary stop-gap measure and periodically audit whether it is still necessary.