Go to AdminCP > Forum Listings Display Options > Depth of Sub-Forums, set it to 1, and that's all there is to it!

This does not really qualify as a tutorial, this is just an answer to a support question. A tutorial would be nice to have a small introduction. Explaining for example what a sub-forum is. Why one might want to display it. What the benefits might be .. a visual perhaps like a screenshot of the setting and/or the result. That all will make it indepth- and qualify as a nice little mini tutorial. Perhaps you could give it a small update before I make it public?

The same goes for this one: http://www.vbulletin-tutorials.com/showthread.php?t=29