Lots of software already on the market claims to stop pop-unders. I don't think it's that much different from pop-ups?

Here's a free one that says it also stops pop-unders:

http://pcworld.com/downloads/file/fi...scription.html

They are obnoxious. I stay away from the sites that have them, themselves. If you're going to use pop-ups or pop-unders or other obnoxious content (too many side ads so your site looks more like an ad site than a reference) then I ain't coming back.