For users on the Premium and Enterprise plans, it's possible to restrict access to contact lists within the CRM. This can be done with both regular contact lists and Media Database contact lists.
Check the 'Restrict user access' checkbox when you create a new list, or when you edit an existing one.
When access to a contact list is restricted, only users with access can see the contents of the contact list and make changes. The list will not show up for users that do not have access.
In the campaign statistics view the list will be visible with an obfuscated title for users that do not have access to it.
Restricting access to a contact list has no effect on the individual contacts in it; those contacts can still be access through other means, for example other contact lists or the search function.