Block countries, seconded

"DENY from [IP range of China, Japan, Spain, Tiscali, Latin America, Quebec, Korea, Thailand, and if you don't expect traffic from the USA, them too]"

Ohhh yes, it works and the horses are pretty much alive. We run our own mail server and 99.9% of our business is done in the UK, and this is exactly what we do. Poland and East European countries included and our spam near zero. Any contacts within the excluded range is whitelisted.

Most if not all UK and West European ISPs are pretty diligent about enforcing open relays and use RBLs so almost all mail traffic through their servers is ham.

It's a wonderful life.