I agree that spam is a big problem but people are overloaded with login/registration requirements for every site they want to visit so we had a discussion a year or so back and decided to open it up to anonymous comments.   We have to have links because it's a science site - people reference - but for anonymous people the links have the nofollow attribute.   So they are basically useless but still annoying.

When we get spam links like you mentioned we ban the IP address but if someone is determined to fill out the captcha and leave a comment, they will do it and it will stick until a moderator removes it.   Bots obviously don't work - we get hundreds of those per day trying to get in - but  a captcha designed to work for people will be manipulated by an actual person.

Generally, I think we could ban all of China and not have any drop in readership but have a huge drop in spam.  That seems a little heavy-handed, though.

If anyone who reads this is an expert in spam deterrents and wants to pitch in and help, send me an email.