The blog and default templates are the Page Templates included in Genesis, but I'm referring to the various Genesis Layouts that can be selected below the post editor area. They have light gray icons that represent the various content/sidebar layout options available for that Child Theme.
If they've disabled the Full Width Content layout then you should be able to re-enable it by using the following code inside a Custom Hook Box:
add_filter( 'genesis_pre_get_option_site_layout', '__genesis_return_full_width_content' );
Just create a Custom Label, name it something like "Full Width Content", then once created you should be able to assign that your Custom Hook Box with the above code. Then got to that page editor screen again and select your "Full Width Content" Custom Label and see if that doesn't force the no sidebar layout that you're going for.