Disable creating new taxonomy terms in term reference field
The content taxonomy module adds additional functionality to prevent new terms with the autocomplete widget.


  D7 and Content Taxonomy 
  
  The module provides extensions to Drupal's
  core Taxonomy Term Reference Fields. At the moment, following
  extensions are available:
  
  
  Parent term selector in the field settings (for options lists) 
  Options for new terms in autocomplete (Autocomplete Deluxe widget supported as
  well): 
  
  Allow and insert new terms (default behaviour)
  Allow and insert new terms into a separate vocabulary 
  Deny any new terms