So, that isn't going to work. However, when XP/SP2 comes out, it will have a Policy Setting called Restrict Internet communication, which will restrict all Internet Communication. This MIGHT be more heavy-handed than what you are after. But, give it a shot. It will be found in Administrative Templates | System | Internet Communication Management.
You might also try control via Internet Zones. Check out: http://support.microsoft.com/default.aspx?scid=kb;en-us;182569 for more assistance.
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