I m using FC10 in my laptop.
That release is out of date. We are at f13 now. note that there has been no fedora core
for a long time.
The recommended way to stop people accessing a website is to ask them nicely not to, explaining your reasons. Listen to mhat they say and if they object, don't let them use your computer. This is not always practical.
It is easy to block a url by aliasing it to localhost in /etc/hosts ... but you want to access the url yourself.
You can drop outgoing requests to a url in iptables, and have a different set of rules when you log in.
For filtering, the usual approach is via dansguardian and, indeed, squid - see below.