I was looking at the code to implement the function to deny access to create a new term, then looking at the module content_taxonomy functions I see that this module got this option.
Looking the code I found a really easy way to implement this function into active tag.

Install content_taxonomy Module

Open the file content_taxonomy_autocomplete.module from http://drupal.org/project/content_taxonomy module

add this || $instance['widget']['type'] == 'active_tags_taxonomy_autocomplete' as follow:

on line 25
from this if ($instance['widget']['type'] == 'taxonomy_autocomplete' || $instance['widget']['type'] == 'autocomplete_deluxe_taxonomy' ) {

become this if ($instance['widget']['type'] == 'taxonomy_autocomplete' || $instance['widget']['type'] == 'autocomplete_deluxe_taxonomy' || $instance['widget']['type'] == 'active_tags_taxonomy_autocomplete') {

on line 68
from this if($instance['widget']['type'] == 'autocomplete_deluxe_taxonomy' ) {

become this if($instance['widget']['type'] == 'autocomplete_deluxe_taxonomy' || $instance['widget']['type'] == 'active_tags_taxonomy_autocomplete') {

on line 86
from this if (($instance['widget']['type'] == 'taxonomy_autocomplete' || $instance['widget']['type'] == 'autocomplete_deluxe_taxonomy' )

to this if (($instance['widget']['type'] == 'taxonomy_autocomplete' || $instance['widget']['type'] == 'autocomplete_deluxe_taxonomy' || $instance['widget']['type'] == 'active_tags_taxonomy_autocomplete')

Now if you go to your content type > manage fields > Active Tag edit, you will see the option to allow or deny new term creation.

Hope this will help somebody

PS. This modify the code of content_taxonomy modules, so take a note of this, because when you will update this module you will loose your code