i don't know how you stop them appearing altogether as they are usually paid-for adverts, but i just click in the top left hand corner of the pop-up page and close it down before it opens up, saving time and annoyance!
Ignore my answer from last week! I've found out how you can do this. Pop up Stopper from a company called Panic Systems is apparently very easy to use, according to Web User magazine, and gets rid of all those annoying pop-ups. It can be downloaded from www.panicware.com/product_downloads.html