A lot of people are using paid captcha solving services to bypass them anyway. Â I think captchas are dumb and ruin the user experience but until someone comes up with something better I guess that's what we're stuck with.
Actually Spam comments not came automatically, What i learn? its these free plugins which give information of blog to owner of plugin and owner started to send spam comments. 99% of plugins doing this. Actually makers of defensive system attacking blogs. Big problem is that Wordpress isn't making any defensive system for this. This is abig problem and wordpress should think about this.   Â
You can use services such as decaptcher to break captchas, Best to go with anti-bot measures such as "answer this question" or "1+1 = ?" or "Which picture is a bird?" : p