You can block receiving messages from people who are not on your friends list from your console's communication settings. See the Support section of Playstation's website to learn how. Bungie.net does not have a setting to prevent receiving messages from all other users. To prevent someone from messaging you on the Bungie app/website, you would need to go to their profile and click the Mute button to block them from messaging. Though that's a very extreme measure to block every person you come across just to prevent the possibility they might send you a private message. Otherwise when someone does send you a private message that you find offensive or hateful, please report it, which will also result in blocking them from being able to message you further. Ninjas investigate private message reports and can restrict users from the ability to send messages and/or comment on the forums if the user violates the Code of Conduct.