In the left-hand navigation bar, click Policies -> Sharing to display sharing settings.
Under File and folder links, ensure that the default link type is set to Specific people (only the people the user specifies).
End-User Impact
Level: Low
With this setting enabled, users will have to specify users that can access the link. If the link is forwarded to other users internally or externally, those users will not be able to access the link.