ok..
mkdir -p /etc/e-smith/templates-custom/etc/squid/squid.conf
cd /etc/e-smith/templates-custom/etc/squid/squid.conf
nano 20ACL40bannedsites
the content must be:
acl bannedsites dstdomain .facebook.com
save and exit, then
nano 40http_access15denyconnectBannedsites
the content must be:
http_access deny CONNECT bannedsites
save and exit, then
signal-event proxy-update
tested on SME 8.1, working for me.. in the first fragment you can add, on the same line, as many domain as you need, separated with a space