This plugin allows you to set default layouts (full width content, sidebar-content, content-sidebar, etc) for different post types (pages, posts, etc) and archives (blog, categories, tags, etc).
Note: you must be using a Genesis theme in order to use this plugin.
To make your pages use the full width (no sidebar) layout and your posts and blog page use a sidebar layout, do the following. In the Genesis > Theme Settings menu, choose a default layout of “Content-Sidebar”. Then, in the Genesis > Layout Extras menu, go to the “Layouts for WordPress Default Cases” section. Under the sub-section “Global Layouts for Singular Pages”, set “Page Page Layout:” to “Full Width Content”.
More information: http://wordpress.org/plugins/genesis-layout-extras/