Somewhere within the My Book Live forum and this forum I discussed and wrote a way of making the Public share read-only without breaking any of the features of the Duo, so it can be done as I’ve done it. 
Luckily I kept a note saved on the my Duo. It goes thus . . . .
Yes you can restrict the `Public` share for the NFS, AFP, FTP and SMB access.
You need to access the MBL's Linux's command line. Instructions are littered through this community.
Using the nano text editor edit /etc/trustees.conf find ...
/shares/Public:*:RWBEX:*:CU
... change this to ...
/shares/Public:admin:RWBEX:*:CU
Do the same change to /tmp/trustees.mod. Within this file if you see `admin:RWBEX` for Public within the DENY LIST then remove it from the appropriate line. Make sure there is one : and not :: if you do this.
Issue the following command:
settrustees -f /tmp/trustees.mod
The `Public` share and directory is now only accessible to the owner/admin user.
This will persist through any user and share changes on the Dashboard UI. If you do any changes to `Public` share using the Dashboard UI you will need to repeat this procedure.
This gives the admin full rights to the Public folder/share and all other users should be denied access.
Hope this helps and you mess and make changes at your own risk.
Personally I have no clue why WD has been to stuborn with the issue of the Public folder. WD’s still being stuborn about this even now. 
My method makes the mimimal change to the back-end of the Duo. I believe there are back-end processes that rely on Public being present.
