How can I restrict access to categories in Help Desk?

To restrict access to categories in Help Desk you can use one of these possible options:

Solution Option 1

Set up multiple business units where categories can be managed through business units.

Navigation: Setup and Maintenance > Service > Business Units > Manage HR Help Desk Service Categories for Business Unit

See: How are business units set up for Service and Help Desk?

Solution Option 2

Change Data Security Policies (DSP).
Caution:
You can't change Oracle predefined data security policies. You must create your own. Be very careful when creating Data Security Policies.

See: How do I define system-access-group-based data security policies?

How do I define access-group-based data security policies?