[support] CCK field permission

Drupal 6.20 – I want a user to be able to select a node reference for a Content Type, but, once set, they can’t go back and edit it again. I’m actually using URL Node Reference, which works great, but if I don’t allow editing of that field, it doesn’t save the Node reference, but if I allow editing of the field, adding works fine, but when they go back and edit the Content, the Node reference comes up. I have the Content Type set to 404 if Argument isn’t present, and the same behavior on editing, but it doesn’t seem to matter on editing. Any ideas how I might prevent my users from changing that node reference? Joel