In WordPress under Appearance > Menus there is the visual menu builder where you can "indent" items to make them sub items of another menu item. I would like to disable the ability to make items sub items by either restricting the ability to drag items inwards or disable any other way that creates sub menu items.

I don't have any code I've tried yet, I've been searching for this for hours and, maybe my search terms are off, but the only articles I can find are to disable a parent item from being a link.

Any help would be greatly appricated! Thanks!