Blocking blacklisted sites for Internet users

In Windows:

In Windows sites are restricted through browser settings or the firewall settings:
IE browser setting:
In the Menu bar select Tools ->Internet Options (as shown in fig-1)

                                       (fig-1)

Now form Internet Options dialog select Security -> Restricted sites -> Sites (as shown in fig-2)

                           (fig-2)

Now enter the url of the blacklisted sites and click Add (as shown in fig-3)

                         (fig-3)

Now click close->Apply ->OK.

You can also make entry in %systemroot%/system32/driver/etc/hosts file as below:
127.0.0.1       www.xyz.com

In Linux (Ubuntu /RHEL):

Its very simple to restrict a site in linux based system.

gedit or vi /etc/hosts and make the following enrty:

0.0.0.0     www.orkut.com (any site you want to restrict)
0.0.0.0     www.facebook.com

or
127.0.0.1   www.xyz.com