First of all, you must be sure this is really automated spam. As it was discussed some time ago
, Chinese spammers can spam a lot manually and there is no work around it, except banning their IP addresses, preferably the whole sub-network.
Also, you can experiment with Captcha settings and set different fonts, different characters number amount, different colors etc. If somebody found a way to break the visual implementation (though I really doubt it), you can always set up the new Captcha layout and that way let the intruders to work on a different algorithm.