Tail, it occurs in the "full" version of the site when using an iPhone.
If you aren't quick enough to hit the back button when an ad comes up, double click the main button, close the browser, reopen the browser and then you'll have the short chance to hit back, if not then close and reopen. You might have to try and hit the back button rapidly and repeatedly.
Deleting files etc doesn't stop the ads from popping up.
So far most of these sorts of pop ups only occur on the phones rather than tablets or computers.