You can block another person from sending you scraps, messages, invitations, friend requests, etc., by ignoring that user. Ignoring someone is a private action, so other users won't know if they were ignored by you.
To ignore someone, just visit their profile and click ignore user on the left menu. Once you ignore someone you'll see a red mark over their profile picture, to remind you that you're ignoring the person.
You can later unblock someone by visiting their profile. Simply click unignore user.