How to use the widget area to extend the page content?

Hook into register_sidebar and remove the sidebar from the global array $wp_registered_sidebars:

add_action( 'register_sidebar', function( $sidebar )
{
    if ( 'lateral' === $sidebar[ 'name' ] )
        unset( $GLOBALS['wp_registered_sidebars'][ $sidebar[ 'id' ] ] );
});

Then adjust the stylesheet or add extra CSS to wp_head to let the page content take the full width.

See also: Change a sidebar name?