In each forum, down at the pottom of the page, there is a menu to change how the forum is viewed. The default is "original post date". If you change it to "last post date" then threads that are responded to will automatically be bumped to the top of the page. Like a *normal* chat board.
ETA --> and you have to click "Apply" after you make the change. And it only changes it for this board. You have to do it separately for each board.