have had this myself, and believe it to be sloppy programming in some of the advertisements. Switch to a window with command lines, select TOOLS, and CLEAR PRIVATE DATA, all of it, in particular 'temporary internet files'. Set your privacy options to bounce pop-ups and not accept cookies from strangers.
Most adverts are OK or informative, but some (MECCA and 888) are highly intrusive. Or SUBSCRIBE, it's not expensive, I will get round to that myself once I gain confidence with online payment methods.