I'm using Drupal 7. Right now, all users can create certain content. the way I have it set it is that all content has to be approved by users of the "editor" and "administrator" roles. Once the content is edited and approved I want to lock out the author so that he can not edit it anymore.
I've not used it, but the Content Lock module might fit your needs!
EDIT AFTER COMMENT
What you could do then is utilise Taxonomy Access Control using the following method:
1) Add a Taxonomy vocabulary called "Locked" (or something like that)
2) Add two terms to that vocabulary - Locked and Unlocked
3) Set the permissions for authenticated users to be able to add/edit the content, but do not allow them to set the taxonomy terms.
4) Set the permissions using TAC so that if a node has the term of Unlocked, it is still editable, but as soon as that term gets changed to Locked, then authenticated users cannot edit that node.
5) Once the content has been submitted and approved, the user with the Editor or Admin role sets the term on the node to Locked. This will prevent the user from being able to access the edit page for the node.
It sounds much more complicated than it really is. You'll see what I mean when you get to the settings page for TAC.
I've done similarly for my article directory (http://www.articlesdirectoryindia.com) . I use http://drupal.org/project/rules to redirect to error page if author tries to re-edit an (submitted and) approved article. I suppose approval is indicated using CCK field in your case.
Otherwise you can simply write form_alter and attach a validation function to give error in this case( trying to re-edit or save an approved article), which is better way to do it.