Topic: Help me fight spambots

I notice spambots often visit the forum and the next thing they do after they are on the index page is visit the last modified topic.
I am trying to use this to fight them by trying to recognize this and ban the ip.
But I would like to know how they get this last modified topic from the forum index because I don't see a link to that topic on the index page. There are links to the last post in each subforum but not one link to the last modified one.

So I'm baffled, how do these bots figure out the last modified topic from visiting the index page??
Hope you understand the question.