Is it possible to do so in NS7?
I used ipfire before, I liked this function.
http://wiki.ipfire.org/en/configuration/network/proxy/wui_conf/clients
Web browser Enable browser check = (acl with_allowed_useragents browser) offers the possibility to allow only specific client software. These include not only browsers but e.g. update or multimedia applications that use HTTP or the HTTPS. If this option is enabled, only the selected clients are allowed by the proxy server. All other requests are blocked.