Hi! That didn't work.
In the global settings, I went into Layout & Design, and chose Content Left; Sidebar Right (so that I could have my sidebar on this page: http://staging.theantgroup.com/the-informant/.
Then I added the code you provided - .single-post .x-sidebar {display: none !important;} - to the Theme Options CSS, but my posts are still leaving room for the a right sidebar anyway (http://staging.theantgroup.com/the-ant-group-honored-as-dream-design-firm/ ).
It's not affecting my old posts (made with X+CS), and it's not affecting my WP-based posts. It's really just the new post I've added using PRO. But I plan to post more using PRO, so I do this resolved pretty quickly (client deadline).
Did I misunderstand your instructions? My blog settings look like yours except I'm using the Masonry style. What am I doing wrong? Thanks! 