The pop-ups are generated by malware on your computer. The free version of Malwarebytes Anti-Malware (which will get rid of it) can be downloaded from here:
https://www.malwarebytes.org/free/
When installing it, take care to de-select the option to install a free trial of the Premium version. Use the 'quick scan' option to run it but don't be surprised if such a scan still takes over half an hour. When the program reports that it's found malware, click the button to view the results, ensure that there's a tick alongside everything found and click the button to remove it. (If there's a really long list of stuff found you don't have to tick each item individually. Just right-click on the first item and look for 'Check all'. Then click the button to get rid of it all)