Actually, I'm using more than one solution now. The deny solution was for the people who kept repeatedly trying to post junk. My site is on a shared server, so iptables isn't a possible solution.

This combination of settings has brought my site's spam to 0.00%.

• CAPTCHA
• reCAPTCHA
• Akismet
• Troll (using SPEWS 2 and both Okean blacklists)
• mod_security
• comment moderation (I haven't gotten any spam in the queue since I enabled the above, but it's just in case)