In the upper left corner of the main window near the name template, there should be a dropdown list that has "sync with LR Mobile" as the first option. From there you should be able to turn it off.
Yes indeed there is a drop down menu, but it has nothing to do with the pop up. I'm not signed in and never I was. The pop up is only related to my internet connection. Is the laptop connected the pop up will not appear. So, Adobe always would like to know what I do, they can't if the laptop is not connected and they are not happy if people do not give away their data for free usage. I hate spy programs on my computer.
I understand your concern.
I will look more into that about the disabling of that screen.