D. Levine on "capcha for comments"

I'm getting a lot of spam comments that are being caught as spam but take up a lot of space until I can delete them permanently. How do I put a capcha on comments so that there will be less of them?
Here is an example of my articles: http://www.americandiversityreport.com/footprints-in-paris-by-poonam-a-chawla/