If the user is clicking on the ad and then lands on your website, the URL will have a query parameter associated with your campaign. If the user bookmarks this URL and uses it every time he visits your website the query parameter will be loaded on every page load.  In this case, Google Analytics will attribute every session to Google Ads and hence even if the campaign is paused or deleted you see sessions coming. Another reason could be if the user has shared the landing page URL to some other places (social media) and different users are accessing that URL to visit your website.  In this case, the URL shared by the user had query parameters in it. And hence even if the campaign is paused or deleted you are seeing sessions coming from it.