YAWASP (Yet Another WordPress Anti-Spam Plugin)
http://wordpress.org/extend/plugins/yawasp/
This plugin uses CSS only, which nearly all browsers support and have turned on. The input fields are given uses randomized names and a hidden blank input field is placed in the form. Bots that are programmed to look for the standard form names are blocked because they can’t find the fields they’re supposed to fill out and give up. Bots that blindly fill in the input fields fill out the blank one by mistake and also get blocked.
Pros: This does not require JavaScript, defeats smart bots that look for standard form names and defeats bots that blindly fill out the form. Humans on the other hand won’t have a problem. Humans don’t need to do anything special to get their post submitted (no CAPTCHA images to debabilze and enter into a field etc.).
Cons: The comments about the plugin from its users indicate that it may be a bit tricky to get it installed and running, but no more so than some other WordPress plug-ins.
And that’s the wrapup of three of the top comment spam blocking plug-ins for wordpress.
Good luck!