As I do not use Libre.fm, I'd like to remove it from the sidepane in Rhythmbox (Ubuntu Precise). I did it some time ago on of my machines. But now I cannot recall how I did it. :-( I could not find a plugin to deactivate, or a package to uninstall.
SnapOverflow
Libre.fm is part of the LastFM plugin
Choose Edit - plugins and click of the Last.fm plugin. You then will have the preferences button visible. From the popup-dialog you can hide/show both services.