Collections displayed for a user in the Content Library can be configured by the company Admin. Apart from the default collection, to display only one collection for a user in the Content Library, go to Organization Settings > Content Library > Collections and assign only a single collection to that user. Only contents from the assigned collection (and the default collection) will be displayed to the user.
The Super Admin or an Admin with access to Organization Settings can create a collection accessible to all users in the company. In such cases, the Super Admin or Admin with All Permissions can remove a particular user from that collection.