Note for Drupal 7 users: this module is Drupal 6 only, you can try out the Taxonomy access fix module instead.
Taxonomy term lock adds a permission for each vocabulary, this permission allow users which does not have the "administer taxonomy" permission to add and reorder terms for this vocabulary.
Users with "administer taxonomy" permission will be able to lock some terms, prevent editing and prevent node to be tagged with those terms.
Locking is an optional feature, you can just use the module to assign individual administer terms for vocabulary N permissions (N is the id of that vocabulary, permissions strings uses the vid in order to ensure consistency if vocabulary name changes).