This reduced the number of "people" registering and then posting, but it has not stopped them from posting as guests. So I took the nuclear option.
.cn is now banned.
I checked, there are two members with a .cn mail address. One is banned already and the other has never logged in since account creation in 2010. I don't think at this point it is going to affect a major number of users, if any. If there is a problem, I will revert the change and find some other method.