If you need to give limited access to users on LAN, you can use free proxy software from
http://www.handcraftedsoftware.org
This is a great software, you can have good control on your network and see who is accessing which web site.
There is a simple way to disable access to a web site on all users on the LAN. This is done by editing hosts file on the main PC that connect to internet, normally one with IP 192.168.0.1
On Windows
[code lang="bash"]
C\:WINDOWS\system32\drivers\etc\hosts
[/code]
On Linux
[code lang="bash"]
/etc/hosts
[/code]
Add following
[code lang="bash"]
127.0.0.1 yahoo.com
[/code]
Once this is done, no one on the network will be able to access yahoo.com.
Repeat the same for all sites you need to block.
yahoo is just an example, don’t block yahoo 
Related posts:
need help. need to block some sites at the office.
we really don’t have a main computer. all are just connected to the router LAN.
need to know easy way to block & unblock sites.
how can I block perticular site on lan connected through main modem and router.
we donot have any main server or pc through which internet is shared.
internet is accessed through default geteway on main modem.
Some modems have a blocking option. If not, you need to block by editing hosts file on each computer on your network. I don’t know if there any other easy solution available.