Because of the spam and constant server attacks from servers in Korea and China, we've actually blocked all of the IP's originating from both places at the firewall on 4 of 6 servers.
Server attacks dropped 80% the first week. Server loads dropped appreciably.
Good resource for those with similar issues:
Okean - Spam Intro
We reload iptables with an updated list from OKEAN every day.
I hope the necessity of doing this changes, but in 6 years it has got nothing but worse.
Until the vast majority of IIS servers over there start running legal (hence, patchable) software, the problem will never get any better. The machines are like swiss cheese.