How to Remove Disqus footer from webpage

You can use negative margins and overflow hidden.

#disqus_thread {
  overflow: hidden;

  iframe {
    margin-bottom: -54px;
  }
}

Outside of the technical workarounds, there is a legitimate way to do this. According to the website, you can remove this with a Pro subscription, which is around $100/month at the time of this answer.

https://<accountname>.disqus.com/admin/settings/general/

I was under the impression that the new disqus uses an iframe and therefore custom CSS can't be applied:

http://help.disqus.com/customer/portal/articles/526768-introducing-the-new-disqus-and-f-a-q-

For the most part, customization is different in the new Disqus because we decided to completely re-implement our commenting embed inside of an iframe. This iframe is hosted on disqus.com and, as such, the browser won't let your website apply styles to it using CSS statements.

In your CSS add:

#disqus_thread {
     position: relative;
}
#disqus_thread:after {
     content: "";
     display: block;
     height: 55px;
     width: 100%;
     position: absolute;
     bottom: 0;
     background: white;
}