How to prevent Google Drive File Stream from updating itself without permission?
I am answering this myself from my rather limited experience because there have been no responses and there is almost nothing about this issue on the web right now.
Google decided to merge the enterprise version of its desktop streaming app (Google Drive File Stream) into its personal desktop streaming app (Google Drive).   They did this by having their Drive Stream app upgrade itself without permission to the new Google Drive.  Nice.  But the new version does not work with El Capitan.  No warning, no instructions, nothing.
Several times, I re-installed the previous version, which would work until it caught on to what I was doing and would again upgrade itself into un-usability. I was hoping to find a way to stop it from doing so, but eventually had to give up.
I upgraded the OS on that computer to Sierra, which took quite a bit of doing, as it was one of the 2011 Macs with the overheated/blown graphics display processor which had to be hacked into working at all.  It took me days to sort out, and only to get this one app to run.
It bears stating that Google does not appear to honor Mac protocol in setting up their desktop streaming clients. The app doesn't appear in the dock.  It doesn't appear in the Force Quit window either, so when it hangs up, which in my case would hang up the entire OS, you can't get out of the program without launching Activity Monitor and force-quitting several unannounced background Google apps. It puts an icon in the menu bar without option and also puts an icon on the desktop without option (when running).
So in short, the answer to this problem is you just have to eat it and upgrade your OS to at least Sierra, because their new app combo is not compatible with El Capitan.  I still think that someone could come up with a fix for this, perhaps by hacking the app to tell the Mac OS that it will run on El Capitan, (because perhaps it could) or on fixing the app so it won't update itself without permission, its doing of which, to my mind, is outrageous.
I would welcome any comments at all or any methods that could be used to restore a Mac user's proper control of the situation.  Thank you for listening!