Your specific use case is going to determine how you do it - we've employed a couple different methods.
Whether or not a Category is included in the OOTB Categories widget depends on if the current user has access to any catalog items within the category AND the catalog item is available via the portal. So the trick is to "hide" the category or all the items in the category.
Approach 1: Hide the Catalog Items from the Portal
When we rolled out our Portal, our target audience was non-IT users so we didn't include our technical/IT centric categories & catalog items (servers, firewall, etc.) by checking the Hide on Service Portal flag option.
The flag is available, but hidden. Navigate to Service Catalog > Catalog Definitions > Maintain Items, and either add it to your list view and set it there, or add it to the form itself.
Approach 2: Hide Categories from Specific Users using User Criteria
We have another use case where we wanted to limit certain users to only have access to a very small number of catalog items. The Available for and Not Available for related lists can be used to control access both Catalog Items, and Categories. To use with Categories, you'll need to add the related lists to the form.
For this, we created a new category just for them and added all of the items they needed to it.
- We did this via the Categories related list on the Catalog Item - the items in question reside in two categories. The ones everyone sees and the one needed for the select group
Then we created two user criteria - "them", and "everyone but them".
- The new category is Available for "them", and Not Available for "everyone but them".
- All the other categories have the reverse - Available for "everyone but them" and Not Available for "them".
It's a little hokey, but we wanted a solution that didn't involve customized widgets.
