Are custom searches possible? (omit a particular forum)

I want my forum searches to omit a particular sub-forum that I have.

Why?
I have a sub-forum that is set aside for people to argue in so that they don't ruin the main forums with arguments. However, many of my visitors use the "Show Recent Posts" option to view the forums. This means forum visitors still see the argumentative topics and I don't want that to happen. It ruins the good vibe.

So is there a way to omit the arguments sub-forum from the "recent posts" search results page?

Thanks
Mark