The Public Share folder is open to all who can access the My Cloud. One cannot disable the Public Access to the Public Share through the My Cloud Dashboard. Each individual user created Share on the My Cloud can either be set to Public Access or Private Access.
If you have not already done so, you should read through the My Cloud User Manual (http://www.wdc.com/wdproducts/library/?id=439&type=25). that manual contains much information on how to use the features of the My Cloud.
If you plan on storing media to the Public Share folder for media streaming, it is generally recommend not to do so. More explanation on that recommendation can be found in the following thread.
http://community.wd.com/t/faq-twonky-dlna-media-server-setup-use/95373
