Sometimes you want to stop certain people from seeing your private Facebook posts, especially if they are merely acquaintances and not close friends. You can do this by unfriending the person. Other times a relationship has deteriorated so much in real life that you no longer wish to interact with a person on Facebook at all. For example, if an ex starts to display stalker-like tendencies, you would want to stop him from contacting you completely, or "block" him on the social network.
If you unfriend someone, you can still see each other's public posts and everything on the other person's profile that she made public, including photos and updates. However, when you block a user, you automatically unfriend him and neither of you can see each other's posts, public or otherwise. The only place that person's post may still show up is in groups or pages you are following. A less drastic alternative to unfriending is to stop a friend from seeing certain posts by using the Audience Selector tool when you post, or by adding him to your Restricted list. Friends on your Restricted list remain Facebook friends, but can only see your public posts.
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It is possible to still message someone who is no longer on your friends list and you may also receive messages from that person in your Others inbox. You don't receive a notification if that person messages you, which would have been the case if you were friends. If you have, however, permanently blocked someone, it's not possible to have a conversation with that user on Facebook.
Adding as a Friend
If you have unfriended someone, you can add that person as a friend later on again. If you block a user, this is no longer possible, and neither of you show up in each other's searches. It will be as if that person does not exist on Facebook. It is possible to unblock someone, after which you will be able to add her as a friend again.
Both blocking and unfriending prevent another person from sending you invites. It is however not necessary to take such drastic measures if you are merely annoyed by all of the invites that person sends you. Via your Facebook settings, you can block all app invites or all event invites from a specific person. This prevents you from receiving any invites from that specific friend, without having to unfriend him.