What is the best way to restrict access to your dedicated server from countries that are known to produce most spammers? I'm thinking most Asian countries...
You may install on your machine, csf and then block the required range of ips on them. You can either block a range of ips or a couple of ips. Let me know if you require some additional help and we are glad to provide.
Check out this site: http://www.countryipblocks.net/ I use shorewall as a front-end to iptables to block access from certain Ip addresses or IP blocks.
I wrote a blog post with exactly what you are looking for: http://lazyscripter.com/2010/02/how-to-allow-only-specific-countries-with-csf/