For anyone interested, I'll answer my own question? After discovering the offensive cookie was coming from 'webstats4U.com', I added it to my IE 'block cookies' list and that solved the problem. In the interim, I received a great message from a PBase user, recommending a free 'no ads' program,
http://www.southbaypc.com/NoAds/
I've not tried it yet but it sounds like it will eliminate the step of trying to find the offensive cookie, by reconizing the source of the pop-up and blocking it.
Doesn't it seem ludicrous for companies like 'webstats' to produce such an obnoxious program that, sooner or later, everyone blocks it? Now the stats are skewed downwards. And worse, some folk like me, often just avoid the PBase site altogether. Where's the gain for their customer base there? Oh well. Thanks to 'Tom' for cluing me in on 'no ads' software. Hope this might be helpful to others, also?