I don't want listings to be shown underneath every category in Main Index. How can I do that?

In order to hide all the listings under each top level categories, you can go to:

Mosets Tree back-end -> Templates -> [current default template]

set "Number of listings under top level cats." to 0. Make sure you check whether if you set to use Main Index template for category pages. This configuration will affect the index page and all the category pages that using main index template page.