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/clientsWeb 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.
You need to create a template-custom for squid.conf.
Otherwise you could try Suricata from testing which has some specif user agent rules which can be blocked.
I've found a way, how you can block specific browsers, but not block all except one.
And like giacomo said, do it with a custom template.