UPDATE: You are in control of who can reach you. Open the More menu on anyone profile to Block them (a full two-way cutoff: no messages, mentions, follows, comments, or seeing each other anywhere, and any friendship is removed) or Mute them (quietly hide their posts from your feed without blocking). In Settings you can also choose who can @mention you: everyone, friends only, or no one, and manage your blocked and muted lists. Blocking and muting are private and never affect anyone karma, standing, or reach.