You might want to constrain users in a collaboration to a particular set of catalog items.
In the SRAM portal, you can create a group "src_no_ext_apps" inside the collaboration.
Any user that is added to this group will only be able to use catalog items that have been explicitly allowed for this collaboration.
The more generic, "public" catalog items will not be available, for this user, inside this collaboration.
Of course, if the user is a member of another collaboration, they will be able to use public catalog items through the other collaboration.