People like me keep posting because we know its a scam but that doesn't fix the problem. It is a continuous annoying pop up that prevents you from using Safari. No advice given so far has helped, unless I have missed something.
It is entirely possible that the website has been altered and those JavaScript popups are embedded into the web page. There is nothing you can do to stop that besides avoiding the website. That is what the instructions about Force Quitting Safari and starting up with Shift key held down are for. If the website is compromised, there is nothing you can do besides maybe try to contact the webmaster and report it. It may be that they are complicit, though.
You could try blocking JavaScript, but that would likely render the website useless.
The second cause of perpetual popups is you have installed Adware. That has also been covered ad infinitum.