Using system preference to block a website

The solution is not in System Preferences. I suggest that you download/install the free Malwarebytes for Mac, run it, and then do the following if you are using Safari:

1. Quit Safari if it is running.
2. Press and hold the shift key while launching Safari
3. Safari Preferences
   1. Websites
      1. Pop-up Windows
      2. If the URL appears here for the site that you want to block, select it, and click Remove
   2. Advanced panel
      1. √ Show Develop menu in menu bar
      2. Press option+command+e to clear the browser cache
   3. Extensions panel
      1. Temporariliy disable all extensions
4. Safari menu : Clear History… : all history
5. Quit Safari, and relaunch normally

Between Malwarebytes and the preceding steps, has it stopped the pop-up website issue?

You are welcome.