If a particular user is annoying you, or you don’t like seeing their posts, for whatever reason, you can mute them. When you mute a user account, you will not see their posts in main activity feed, and they cannot message you directly.
Please note that the mute functionality is useful but lightweight. What this means is that you may still see some evidence of the muted account (eg; that they have messaged you, just ignore and delete them), and certain other edge cases. However the main area of nuisance will be gone.