UseCanonicalName is only available in server config files, not in .htaccess. Because of the way that .htaccess works, it's too late to change that setting once the server API processing reaches the .htaccess level.
As stated in the documentation, you'll have to add the UseCanonicalName off directive to the <VirtualHost> container or to a <directory> container within that virtual host.
If you know how to use vi or have emacs or any other editor installed, and have permissions set to allow it, then you'd just change that line in the appropriate <VirtualHost> entries in httpd.conf and restart the server. Otherwise, you'll have to ask the hosting company to do it.