If you have spam comment this may be because you allow unhautenticated anonymous users to post comments.
If so:
1/ Disallow comment for unhautenticated anonymous users. But you probably don't want to do that.
2/ If this is not the case and if the bots are not black bots that pretend themselves to be Mozilla or whatever, an elegant way would be to only tweak the .htaccess file http://en.linuxreviews.org/HOWTO_stop_automated_spam-bots_using_.htaccess
3/ There are other advanced methods, but you'd have to dig into the code, some suggestions there
http://stackoverflow.com/questions/1577918/blocking-comment-spam-without-using-captcha
http://webmasters.stackexchange.com/questions/22624/prevent-spam-using-honey-pots
...
Edit:
Just thought about a more elegant approach of the "honey pot" principle which would be - if your server environment allows it - to have a Web Application Firewall, namelly Mod Security, to handle the false submission (honey pot), instead of putting the load on Q2A for this.
This way you would have very minimal core code hack to do.
More on this here (not tested by myself, but no reason it shouldn't work nicelly)
https://grepular.com/Blocking_Comment_Spam_Using_ModSecurity_and_Hidden_Fields
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