Disable the guest user completely in El Capitan without having to disable 'Find My Mac'
@alan-shutko pointed out very important info about this in his comment:


  The point is to make Find My Mac work. If you turn Find My Mac off,
  the guest account will go away. The reason it's there if Find My Mac
  is on is that if someone steals your mac, it cannot report its
  position unless the thief connects it to a wifi network.


Looks like we should keep it on.