This box is coming from the left-bar setting in the category-level configurations. It is found in the Layout tab. To remove this box from your category pages - or hide the left bar - follow these steps:

From the Admin expand the Catalog tab and click on Categories.

Now choose the sub-category that has the box you want to remove.

After navigating to the category, click on the Layout tab.

Scroll down and check the Hide box under the Left Bar configuration and click the Update button on the bottom right-hand side to save your changes.