If users are locally logged in, you can use iptables:
iptables -A OUTPUT -o EXTERNAL_IF -m owner --uid-owner USERNAME -d DESTINATION_WEB -j REJECT
where:
- EXTERNAL_IF is the name of the Internet-bound interface (e.g. eth0)
- USERNAME is the login id of the restricted user
- DESTINATION_WEB is the DNS name or IP address of the destination website. Beware of sites that host many websites (such as blogger) or those that have multiple public IPs (such as Google)