How to remove a dock from Plank?

Uninstall plank
To remove just plank package itself from Ubuntu 16.04 (Xenial Xerus) execute on terminal:
sudo apt-get remove plank

Uninstall plank and it's dependent packages
To remove the plank package and any other dependant package which are no longer needed from Ubuntu Xenial.
sudo apt-get remove --auto-remove plank

Purging plank
If you also want to delete configuration and/or data files of plank from Ubuntu Xenial then this will work:

sudo apt-get purge plank

To delete configuration and/or data files of plank and it's dependencies from Ubuntu Xenial then execute:

sudo apt-get purge --auto-remove plank

Source: Here

Try to reset its settings in dconf (installable with sudo apt-get install dconf-cli):

dconf reset -f /net/launchpad/plank/

Try to locate Plank in the current user XDG Autostart:

ls .config/autostart/
# then remove plank from here
rm .config/autostart/plank*.desktop