i like the idea of blocking a specific country because the spam origin is very high from specific countries.
This is a little amusing, I bet you that - half the people here offering the concern and advice - are the same culprits pushing link building (blog commenting) and preaching about the effectiveness in other threads... yet they voice their concerns here...
Bots are getting smarter and the script is getting better. On most forums it seems like they break through every now and then if it's been tweaked. I recommend just staying up to date from time to time, if you notice more break through's do some research and you may have to higher restrictions, email confirmations just depending on how bad it is. If a new update has been released for that software update to it! Each site is different, like blogs, comments you just have to find what will work for you.
There is no way of avoiding spam. It is something we all have to manage if we want to use the internet. Unfortunately, blocking specific countries will not work in all cases, because spam often comes from breached servers in the US or otherwise. I really like the point raised by thsadmin! I had to deal with numerous comments in Wordpress (back when I had a blog) and found that many comments say "I like your site" and end up being links to adult websites or a discount pharmacy. Buying links is fine, but spamming for (irrelevant) free links is for losers. Did you ever hear Bill Gates or Steve Jobs say, "Well the thing that really drove my business was advertising adult websites on programming blogs"... probably not.