You need to use the QueryOpen and QueryModeChange events of the form, but you will have to do it in LScript.
In the QueryOpen event, you check if the user is opening the document in edit mode or in read mode. If in edit mode, then you check the field value. If the field is set to prevent editing, set the Continue variable to false. You may wish to pop up a little message stating the the user cannot edit the document at this time (to prevent calls to the helpdesk). Or you can simply revert to read mode, again with a little popup.
In the QueryModeChange event, just check if the field is set to prevent editing. If so, set Continue to false again, and a little popup.
If you have any trouble with coding this, just reply to this thread and we'll work together.
Pascal.
I've got nothing to hide, and I'd very much like to keep that away from prying eyes.