Sometimes, users will not be comfortable with getting messages from entire community and may decide that only friends can send them new messages.
As an admin, you can enable this configuration globally as explained in this article, but users also have the possibility to override this setting by going to their profile, then clicking on “Update Info” button in the cover and switching to “Preferences” tab.
At the very bottom of the page, there will be multiple options regarding chat behavior. Checking the “Allow new messages only from friends ” will take effect immediately and only friends will be able to send messages to that user. Admin user is exception to this rule.