Spammers are bots, always as far as I'm aware. If you want to stop spam you need to prevent them from being able to sign up automatically. I know from experience that most of the traditional captcha's don't work very well. Real people will get more trouble signing up and these bots seem to have little trouble with them at all. Most people hate the traditional captcha's because they're too hard so I wouldn't opt for this.
There are captcha's that are easier though. 4 small pictures that you need to arrange (like a simple puzzle), or a picture you need to match (drag eggs to a basket). I think these are called visual captcha's. They are really easy but because it involves a randomness and dragging something with the mouse, I don't think bots are able to get past them (yet).
Also what seems to help is one or two random questions from a database that need to be answered during the signup process. These questions would be simple for anyone but should not easily be answered by a bot based on dictionary words etc.
Either way, user signup process will get a little bit more difficult and there will always be users bailing out because it doesn't work for them.
Or, you could block the entire ip range of the asian continent
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Or, accept it as part of doing business as suggested by MrC. I think this is only a viable option if the amount of spam is low enough.