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Right click on banner, check "Block images from this server".
That works for servers that do nothing but serve ads such as the evil Doubleclick and Gator or whoever, but AdShield blocks any string you put in, like, a server you don't want to block but which has an ads directory like www.whatever.com/adbanners/ - you can just block /adbanners/ which I really like.
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why don't you just use the HOST file trick?
Eh, again, I'm too used to the ease of AdShield.
What I'm trying now though is using Outpost Firewall's ad blocking plugin. Firebird's cache doesn't seem to be as easy to look in as IE's though - can't tell if I'm blocking the ads before they download.