This won't do anything. The attacks are not from the offending countries they're from botnets of compromised devices.

North Korea doesn't care if you limit their internet they already allow people to go outside their own.

America already limits its upstream to China and Russia through a private companies such as Cloudflare and Spamhaus. It's often the case that for Chinese users seeking to escape censorship, once they've worked their way through the Chinese Great Firewall, they find themselves in front of the American one.