Deal all m0n0wallers

Can you tell me how to block certain web sites from being accessed by the
users. I tried to id the IP address of the DNS servers of the web sites and
put them in the Firewall Rules under block and it still accessibles. Then I
found a forums that says that reject works faster and better, so I tried
reject.

On one computer it works, but on another computer (that has pass through
mac) does not works. Can you tell me what to do to make the filter 100%
working?

Thanks
Rendra