I had this problem a few months ago with my old site, and was thus forced to find a solution that was light-weight, easy to implement, and most importantly was effective. I decided to turn toward my friend CSS to help me out. The idea here is setting up a form with a text field and via CSS making it invisible. Then, if a post is sent to a php script handling the request and that text box has information in it, that means a human didn’t fill it out, and the script is simply aborted.
Interesting, and surprisingly simple method for blocking most comment spam. This will be the most likely solution if I run into any comment spam problems here.
Wandering the Interwebs 