How do you hide the little blank box shown next to the last node of the tree ?
It looks misleading, users are mistaking it for a checkbox.
This icon is defined using CSS. You can override it using ".scheduler_default_tree_image_no_children" selector (for the default CSS theme):
background-image: none !important;
In order to override the default theme, use "!important" or a more specific selector like "#dp .scheduler_default_tree_image_no_children".
You can also generate your own theme using the theme designer and remove the *_tree_image_no_children definition from the generated CSS file: