Pop up adware windows

Block popups.

1. “Force Quit” Safari.

     Press these three keys together: Option, Command, and Esc (Escape).

     https://support.apple.com/en-us/HT201276

2.If your Mac doesn't shut down, try Force Quit to close any apps that aren't responding. 

   If that doesn't work, press and hold the power button until the Mac turns off.

   Press and hold the power button to turn off your Mac only if it isn't responding. 

   You'll lose unsaved changes to any documents that you have open.

    https://support.apple.com/en-us/HT201150

3. Launch Safari holding the shift key down.

4. Block pop-ups when you see pop-ups.

   https://support.apple.com/guide/safari/customize-settings-per-website-ibrw7f78f7fe/mac

5 Check for adware.

    Use  the latest release of Malwarebytes for Mac to remove malware/adware, if installed on your Mac.

    For instructions:  Install Malwarebytes for Mac          Uninstall Malwarebytes for Mac

    Click the “Scan Now” button. Once done, quit Malwarebytes for Mac.

    Restart the computer and relaunch Safari holding the shift key down.

Also check Safari/Preferences/Websites/Pop-up Windows to see if you can block that website there.