I'm surprised that nobody seems to have suggested the most obvious cause of unwanted pop-up ads, which is MALWARE.
Download, install and run the free version of Malwarebytes:
https://www.malwarebytes.org/free/
Points to note:
1. When installing, take care to deselect the option to install free trial of the 'PRO' version ;
2. When running it, use the 'Quick scan' option ;
3. Don't be surprised when that 'quick' scan takes over half an hour to finish ;
4. When it does so, click the button to view the results, ensure that there's a tick alongside everything listed and then click the button to delete what's been found.