Very graphic SPAM in comments that won’t stop!

Lately I’ve been getting A LOT of spam under my comments and it’s all for one post. The spam is VERY graphic and usually contains links to porn websites. I’m not even clear why this post is getting spammed with porn because it doesn’t even contain mature keywords. At first I would just select the spam option in wordpress (which didn’t seem to do anything besides just deleted the post) but it started to get worse, so I downloaded a BAN IP plugin to try and stop it that way. I’ve banned around 100 IPs and I still get about 20 spam comments on this one post every day. Is there anything I can do besides adding that “enter the text shown in the box” comment verification spam tool under each post? I hate those things almost as much as I hate spam and that’s my very last resort.