Sys.Application.add_load(ApplicationLoadHandler)

function ApplicationLoadHandler(sender, args)
{
    if (!Sys.WebForms.PageRequestManager.getInstance().get_isInAsyncPostBack())
        Sys.WebForms.PageRequestManager.getInstance().add_initializeRequest(InitializeRequest);
}

function InitializeRequest(sender, args)
{
    var prm = Sys.WebForms.PageRequestManager.getInstance();

    if (prm.get_isInAsyncPostBack())
        args.set_cancel(true);
}