How to remove Nuvola Player completely?

To remove all user data of Nuvola Player, use command:

rm -rfv $HOME/.local/share/nuvolaplayer $HOME/.config/nuvolaplayer $HOME/.cache/nuvolaplayer

You can also remove Local shared object of Flash plugin (aka Flash cookies):

rm -rfv $HOME/.macromedia/Flash_Player/#SharedObjects/ $HOME/.macromedia/Flash_Player/macromedia.com/support/flashplayer/sys/

If you have installed Nuvola Player from PPA, type
sudo apt-get purge nuvolaplayer