The block page its one CGI used both from SquidGuard and SquidClamav.
You can change it on SquidGuard using this commands (see http://docs.nethserver.org/projects/nethserver-devel/en/latest/content_filter.html):
config setprop squidguard RedirectUrl <url>
signal-event nethserver-squidguard-save
But for SquidClamav you need to create a template-custom of this fragment /etc/e-smith/templates/etc/squidclamav.conf/20redirect.
Finllay, the CGI code can be found here: /var/www/cgi-bin/nethserver-block.cgi