There are no Pop-Up adverts on PPRuNe. If you have a window open to a different weebsite, even if it is reduced to the menu bar it will still be reloading pop-ups from that site.
I repeat, there are no pop-up ads on PPRuNe. Make sure you close windows to other sites you may have visited when you browse PPRuNe and you should not get any pop-up ads.