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Are you talking about blocking local ip addresses on your LAN from accessing the internet or remote ip addresses from accessing your LAN?
If you are talking about blocking someone from sending you messages or accessing services on your computer the 2wire gateway doesn't support this level of fine grained firewall control. You can accomplish this at the machine level however. Gotta love iptables...
Oh... you say you aren't using *nix with iptables? Hmmmm... well I haven't been a Windows user for awhile, but maybe this will work...
If instead you want to keep one particular machine on your LAN from accessing certain services, sites, or web resources then you are going to need some sort of content filtering/access controls... both are available as add-on services for 2wire with some ISPs.
Hope that helps get you started.
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