There is no easy way of doing that in DG3. This is how until we get the time to add a proper method.

The example is taken from the cms viewarticles in the DG3 by example book

There is an extra box here

Open code editor and locate the box in the form elements in the code, mark the element between the to {} and delete

Make sure you press save on top of Code editor

Return to you Design form and the element is gone

Save page in Design form, refresh and preview.