How to remove remaining rectangles from Dashboard

I have installed Ultimate Branding plugin and removed all widgets, but there remains 3 empty dashed rectangles that I don't know how to remove (they seem to be slotes for widgets). How can I remove those?

And another question is: how can I create a custom widget and place it there? The widgets that I can create in appearance seem to have to be specific for certain positions in the page, so when I create a widget I have to initially place it somewhere (sidebar, main page, etc...), but I would like to have it bound to nowhere.

Thank you.