Can I purge configuration files after I've removed the package?

Here is a simple command that will meet your request:

dpkg --purge $(dpkg --get-selections | grep deinstall | cut -f1)

Yes you can.

From the command line:

sudo apt-get remove --purge packagename

This will remove all of the remaining files that the package installed.

You can also do this from a GUI:

Install Synaptic from the Software Center
Run Synaptic
Find packages listed under "Not Installed (residual config)"
Right click the package and click, mark for complete removal
Click the check button on the tool bar and click apply when the dialogue pops up.

You can purge all previously uninstalled packages with aptitude:

aptitude purge ?config-files

You can also purge individual packages but then you would also have to purge the dependencies one by one. It is not practical.

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