If you are using squid then it is quit easy to setup deny filter. Or you can use iptables.
First find out ip address of site such as hotmail.com
and use iptables to append rule to exisitng tables (better in script)
iptables -A OUTPUT -d 18.104.22.168 -j DROP
See for more example http://www.cyberciti.biz/nixcraft/vi...-outgoing.html