Forum access based on role
Not sure what version of Drupal you are using, but have you looked at the Forum Access module?  For D6 there is a stable release and the D7 version is in Beta. The module is also very straight-forward

From the module's page:


  This module changes your forum administration page to allow you to set
  forums private. You can control what user roles can view, edit,
  delete, and post to each forum. You can also give each forum a list of
  users who have administrative access on that forum (AKA moderators).
  
  This module requires the ACL module in order to function. The D7
  version also requires the Chain Menu Access API module.
  
  Forum Access is compatible with the core Forum module, Advanced Forum,
  and Content Access, Domain Access as well as all other well-behaved
  node access modules.