If my memory serves well a while back Guitarman posted about this infernal problem and reccommendations were given on either freeware or shareware. I have my firewall configured to prevent these but I have a few friends who swear by PopupCop it seems to be highly rated by both Cnet and Tucows and has a free 30-day trial then only $19.95 to buy.
Got to agree with Gecko Pop up stopper at www.panicware.com free for the basic package or it used to be just remember to switch them off if you do any internet banking etc, as they tend not to allow the connectionwith coded sites