This has been going on literally for years now. Your response is always "post some code blah blah" and then nothing is ever done about it. Why don't you tell your obviously-shady ad provider that enough is enough and move to a provider that isn't going to blast your users in the face with this obnoxious crap? The animated banners are bad enough but the popup ads are ridiculous.
It's not "my" ad provider (I'm just a mod here; you want the owner, Anand), but all the same the reason is because the ad guys need to accept 3rd party ads to cover the forums. There aren't enough 1st party ads to cover them. And whenever you accept 3rd party ads, crap like this gets through.
If you look at the ad suppliers, it's TribalFusion, MediaPlex, DoubleClick, etc. All of which are legitimate 3rd party ad firms. The problem is that the source of the popups are illegitimate ad buyers who buy a block of ads and then send in an ad laced with a popup.
Whenever I have reported a popup to the AT ad guys they've promptly removed the ad, but of course the illegitimate firms buying the ads will simply buy another one. So the problem is right up there with forum spam: there's no fool-proof solution short of not accepting any ads (new members) at all. But all the same the rest of the mods and I are doing what we can to cut it down, and generally speaking it works.
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