From the Facebook Privacy Checkup page on the web, you can click Who can see what you share to choose what the default setting is for this audience selector. If you want your posts to be seen only by a certain group of people, or by everyone on Facebook except for a few contacts, you can configure this here.
There’s also a section called Limit past posts. If you click the Limit button underneath it, any older posts that were shared with an audience of friends of friends or indeed anyone on the web will be locked down so only your current Facebook friends can view them.
Anyone you’re Facebook friends with can send you a direct message via Facebook Messenger, so if you don’t want this to happen then you need to unfriend or block them. (See below for more details on this.)
Anyone else who’s on Facebook can message you too, but these messages don’t go directly to your inbox: Instead, they go to a message requests folder. On the desktop interface or inside the mobile app, click or tap your profile picture (top left), then pick Message requests to see these messages. You can reply to these messages to continue the conversation, or ignore or delete them.
From the mobile Messenger app, tap your profile picture then Privacy and Message delivery to decide whether friends of friends and non-friends on Facebook can send you message requests at all.
Unfriending and Blocking
Friends that you are connected to on Facebook can contact you much more easily than people who aren’t. To sever a connection with someone, visit their profile page on Facebook and click Friends then Unfriend.
For people who you absolutely do not want to hear from on Facebook, use the block feature. Blocked individuals won’t be able to see what you post on Facebook, they won’t be able to tag you, they won’t be able to message you, and they won’t be able to add you to events. If you block someone, they won’t be notified, but they may notice they can no longer be friends with you or get in touch with you.
To block someone, from the Facebook settings page click Blockingthen choose Edit next to Block users. Select Add to Blocked Listthen enter the name of the person you no longer want to hear from, then choose Block. You can unblock people if you change your mind, but you’ll have to then manually add them as friends again.