This super-annoying thing was happening on my iPad the other night too - I finally just gave up and it hasn't been happening since. I didn't even know iPads were subject to malware; I've never had anything like that pop up before.
The Mods are strangely absent from this thread...I have to assume this is a well-known issue to DIS IT and that there's no fix for it. It's probably some sort of unintended consequence to having the ads on the site in the first place, which must be a revenue source that takes priority over functionality.

Oh well, but I do wonder if the "PIA" factor is contributing to how dull Community Board seems lately. Maybe it's just me...