I installed Cairo-dock using The Software Center, I didn't like it so I uninstalled it. It is still there, it still opens, but it isn't marked as installed anymore.
For future reference, to remove the main Cairo-dock packages from the terminal:
sudo apt-get remove cairo-dock cairo-dock-core
If you want to make sure other packages are removed, you can also add the following:
cairo-dock-plug-ins cairo-dock-plug-ins-data cairo-dock-data