You cant yahoo wont allow its users to be able to use there service.
that is true but i have a small trick to teach,
if you dont want to allow website directories for public as in prevent people to see the website folders contents
you can do the following
create an index.html empty file name its title Access Denied- Restrict Content
than upload a copy of this created file to each folder found inside your yahoo web hosting panel
i suggest you use filezilla and ftp your website and enter each folder and paste the index.html file inside what will happen is
if your for example website name www.xxx.com and you have for example a folder called css www.xxx.com/css people would be able to see whats inside the css folder but if you place an empty index inside the css people wont be able to see your folder contents anymore.
i hope this helped.