Plank is a great simple dock to use.
However I noticed when i added the CPU monitor docklet from preferences it did not allow me to remove it.
I was using Plank 0.11.3.1597 (plank –version)
I had to rename the CPU Monitor config file here to remove it.
~/.config/plank/dock1/launchers
This then removed the CPU Monitor from Plank.
I also had 2 clocks appear at one point. To remove one i found the clock config file and renamed it as above.