Unable to stop autostart on Ubuntu

JC from Dropbox Support provided instructions to resolve this issue by doing a full reinstall and re-sync and it worked for me.  The Start Dropbox on system startup option now works as expected and I can turn off autostart.

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First, make sure you save and quit ALL programs that access files in the Dropbox folder.

Note: Please be sure to make a note or take a screenshot of any Selective Sync settings you may have applied as well as the location of your Dropbox folder (if not in the default location) prior to re-installing, as you will need to match these settings upon re-install. If you're using a headless version of Dropbox you can access the list of Selective Sync folders by running "dropbox exclude".

Depending on your OS and the package you used to perform the installation, you could have files in two different locations. I'm sending you instructions for both of the cases, so if some of the commands error out don't worry.

Run the following commands in your terminal:

dropbox stop
dropbox status # Should report "not running"
rm -rf ~/.dropbox-dist
rm -rf /var/lib/dropbox
rm -rf ~/.dropbox*
sudo apt-get remove nautilus-dropbox
sudo apt-get remove dropbox
rm /etc/apt/source.d/dropbox

Once you're done you have two options:

1) Install the Dropbox application with its graphical components using one of our packages. If you want to do that, download the correct package for your OS and architecture here:

https://www.dropbox.com/install?os=linux

Open your downloads folder and double click on the package file to start the installer.

When Dropbox has finished installing, please sign in and apply any Selective Sync settings you may have had prior to the reinstall and select the Dropbox folder location (if not in the default location) during the sign in process. Your account will take a few moments to reindex the files and sync any pending changes.

2) The second option is to run a headless version of Dropbox. This version will not have any of the menus or graphical components and can only be interacted with through the command line. To install this version do the following:

If your machine is 32-bit:

cd ~ && wget -O - "https://www.dropbox.com/download?plat=lnx.x86" | tar xzf -

If your machine is 64-bit:

cd ~ && wget -O - "https://www.dropbox.com/download?plat=lnx.x86_64" | tar xzf -

Next, run the Dropbox daemon from the newly created .dropbox-dist folder.

~/.dropbox-dist/dropboxd

When Dropbox has finished installing please sign in and apply any Selective Sync settings you may have had prior to the reinstall. Your account will take a few moments to reindex the files and sync any pending changes.

More installation and CLI information is also available here:

https://www.dropbox.com/install

Once that's done, please open the preferences of the application, (via right clicking the tray icon), and in the Accounts tab of preferences, untick "Start Dropbox on system Startup".