Keep Google from spidering specific thread

[REIGN SS]

I couldnt find anything, is there a way to keep the spider from searching a specific thread? I have an automotive forum & use Google AdSense. Ive noticed that the Google AdSense Spider is spending alot of time in my General Off-Topic thread, where everyone just talks about random things that arnt site topic related. I would like to keep the spider out of the thread, as i dont want it to be confused when selecting ads. Any help would be great!!

[Criticize]

Edit your .htaccess file and restrict that from search engines

[REIGN SS]

What would i add to the .htaccess file?

I'm sorry I'm still very new to this...