BLOCK 2 websites on 2 pc's

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Hi, I hav win 2003 server and xp/xp pro clients.I have a ADSL internet line
(dedicated) that is used through the server (proxy server). I need to block 1
website on 2 of the secretaries pc's. Please help as the are using my capping
for the month. I don't want to use the advisor option under int explorer.

Thanks guys!!!! (oeps and girls!!!)
.