As root, edit the /etc/hosts file, add a line for each site to block, including subdomains, like this:
127.0.0.1 hosttoblock.com
127.0.0.1 www.hosttoblock.com
127.0.0.1 mail.hosttoblock.com
Then flush DNS cache:
# service nscd restart
# /etc/init.d/nscd restart
# /etc/init.d/dnsmasq restart
