Hi,

You can use the CSS code below: 

@media screen and (max-width: 600px){
   div#wpd-bubble-wrapper {  
      display: none !important;
   }
}

Insert the code in the "Custom CSS code" textarea, located in the Dashboard > wpDiscuz > Settings > Styles & Colors admin page.

Don't forget to delete the caches before checking.

Also, please try in incognito mode if you're checking on mobile devices. Because mobile browsers have a hard cache, you'll not be able to see the change immediately