Ugh - spammers have figured out how to mess with Safari. Usually you have to hit the back button several times to get off the page.
If the popup keeps coming up then it is probably loaded into memory. Go into safari, or your in app browser if you are accessing the web from within an app, and close every page. If one of the spam pages is still open it will keep loading.
Now go into your iPhones >Settings and scroll down to >Safari and choose block pop ups, fraud website warning and then choose "Clear History and Website Data" - this may cause chaos for you if you rely on cached webpages, passwords, etc. This last step is what resolved the problem for me, but if you don't want to do it restart your phone or use another browser like chrome.
If your safari is set to share browser history between your phone and your mac you should also log into your Mac, goto Safari and close out all the web pages.
