I want to block certain sites for certain persons, i know that if a place the line the site of home.nl is blocked and when i tried to open it i got the 404 error.
But the problem is that nobody on the computer can acces the site.
How can i do that for a certain person.
I have a standard installation of Mandriva 2008, no samba or other stuff installed. I don't want to install samba or other stuff.

I would suggest using squid, and then filter the sites out with this. You can make acl's for squid, and then tie it to a person. I installed OpenLDAP, used squid with the squid_ldap_auth module, and then I could then use the username from this to decide what to allow and what to block.

So then Firefox would be configured to use squid as a proxy server?
But then couldn't each user then just edit the Firefox preferences to connect directly to the internet without going through squid, and could then see any sites they wanted??

You could adjust the system's networking to disallow direct internet access.