NX Removing CDL file inclusion in a post

How do I get the "Include Own CDL File" box unchecked?

That option is enabled if you have (or had) the UDE Editor turned on.  Before opening the post, explicitly turn on the UDE editor using Options; Enable UDE Editor; Yes from the main menu, then open the post.  That should make the "Include Own CDL File" field live so you can check/uncheck the box.