You can uninstall or removes an installed leafnode package itself from Ubuntu 12.04 LTS (Precise Pangolin) through the terminal,
$ sudo apt-get remove leafnode
If you would like to remove leafnode and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove leafnode
If you use with purge options to leafnode package all the configuration and dependent packages will be removed.
$ sudo apt-get purge leafnode
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove leafnode