How can I turn off the default public share?/guest user

WD Legacy Products My Book Live
4

I believe you should look into this file.

The answer to your question lies there : 

/etc/trustees.conf

It describes which folders of the drive are to be shared, and to which users. The Public share is marked to be acessible to anyone.

Warning: only change this file if you really know what you are doing. Also, make a copy before taking any action.