The quickest way to get out of a persistent popup that won’t go away (unless you do what it demands!) is to quit or force quit* the browser then restart Safari holding down the ‘Shift’ key.
Holding down Shift allows Safari (or any other app) to restart without resuming its last state.
While this is a great, fast way to solve the problem, it can be annoying if you had other tabs open, and you don’t want to loose those too (or any unsaved data they may contain).
1. Go to Terminal and paste this command (it’s all one line):
2. Reopen Safari
You’ll get all your tabs back including the hijacked tab, but the pop up won’t appear, and you can now close the hijacked tab.
(alternatively you can do that in Terminal).
Don’t forget this step, or you’ll think the web is broken!
*You can force quit an app by pressing the following keys in combination on your keyboard <command><option><esc> then choosing the app you want to quit.