The category access policy rule can be used to hide specific categories from parts of the organization. This is setup in the purchasing policy and the association of the policy to the organization. This is a quick walk through of how this rule can be setup and what the effect looks like.
I did a post a while back that shows a similar walk through but showing the effect using the Windows Client as the user experience when creating a requisitions. You can find that post here
Cheers
Lachlan