Okk.. well then you should try this command..
but this works only when you have that package installed..
sudo apt-get --purge remove <package name>
suppose you have uninstalled that package and then run this command.. you will get an error..
so,, in order to remove the old configuration files with the package already removed you should probably try this command..
and as i have already mentioned..this works even when the package is not really installed..
sudo dpkg --purge <package name>