This very small module adds an extra layer to forum-select field when editing forum topics.
Adds a new permission,
change topic forum, so users without this permission (or
administer forums) cannot move an existing forum topic to another forum (taxonomy term) (they still need
edit own forum threads or
edit any forum thread permission in order to get the edit form that this module alters ).
This module makes use of a form_alter function to alter the forum's "Leave shadow copy" and "Forums" fields.
Due to rare use-case and experiment code, this module will not be changed to a full project.
It's currently for Drupal 6. But if anyone is interested in a Drupal 7, I'd be happy to upgrade to.
Idea is from this forum post.