Blog comment spam is a big problem. I started out with no protection on this blog. I found out rather quickly that wouldn’t do.
So I implemented a CAPTCHA which requires you to enter a random code to prevent automated comment submissions. Then I found out actual humans were submitting comments (or perhaps very smart anti-CAPTCHA programs).
My next step was disabling auto-approval unless you had one pre-approved comment. But that only resulted in me clearing out spam every day and discouraging real-time discussion because comments wouldn’t show up immediately. After a long time, I finally found Akismet.