You can accomplish this a couple different ways on Volusion. The key is that there are unique elements to both the product and category pages.A simple way to do this is to detect the type of page based on the URL.
Volusion Product Page URLs all contain "-p/" (or alternatively "_p/") Volusion Category Page URLs all contain "-s/" (or alternatively "_s/")
So, you might do something like:
$(document).ready(function(){
var pathname = window.location.pathname;
if(pathname.indexOf('-p/') > -1){
$('body').addClass('product-page');
}
});
Then just add the appropriate CSS with a display:none for the left nav.