Read the rest of the article for a few pointers on various means of disabling them...
Kill all pop-ups no matter which browser you use:
Head over to macwrite.com and read their excellent article on creating and using a "hosts" file in OS X to isolate and block those sites that create the pop-unders.
Mozilla 0.99:
Disabling pop-unders in Mozilla, my personal browser of choice, is amazingly easy. Just open the Preferences panel, then open the Advanced section, and click on "Scripts and Windows". Here you have total control over what the site is allowed to do to your windows. In the "Allow webpages to:" section, simply uncheck the first item, "Open unrequested windows". You can also change the other options to further restrict those items the sites can control. Similar functionality should exist in Netscape 6.2, as it is based on Mozilla.
OmniWeb 4.1b2:
It's similarly easy to disable these ads in OmniWeb. Open the Prefs, go to the JavaScript section, and under "Scripts are allowed to open new windows", select "only in response to a link being clicked".
iCab 2.7E:
I'm not certain of this one, but I think you could do it with Prefs -> InScript -> Filter, then check the "Use InScript filters" and then uncheck the "open windows" option. There are also image blocking filters which you could enable once you've identified the source of the offending ads.
Opera 5.0 preview:
Under Preferences, select "Windows" then uncheck "Allow documents to create windows".
Internet Explorer 5.1:
IE has no pre-defined method of blocking either images or pop-ups, but Larry Rosenstein posted a genearl method of working around this problem on Macintouch today. Adding some detail to Larry's instructions, you need to choose the Security Zones area of the Prefs, then switch the Zone to "Restricted sites zone". Next click on "Custom" for the security setting, and then scroll to the bottom of the next dialog box that appears. In the "Scripting" section, change "Execute scripts" to disabled. Hit "OK", and then select "Add sites..." and enter the URLs for sites that are serving pop-up ads. The downside to this message is that all JavaScript will be disabled when you visit those sites, but it should be the end of the pop-ups.
If you have other thoughts on methods of disabling these annoying ads, please post them...

