Strange thing to want to do. A visitor goes from the main index page to a folder, but then has no way to get back to the main index page. In site navigation that's referred to as a "trap door," and there's not much call for it because it produces an unpleasant visitor experience.
There's no way to apply different settings to different folders - the page coding for one folder is the same as for every other folder in the album. Each folder doesn't have its own script. But you can disable the "up one level" buttons for an entire project. Go to
Tiger > Advanced > Custom code > CSS and enter:
.icon-one-level-up {display: none;}
