For example, on my Squeeze VM:
$ grep-status -n -sPackage -FStatus config-files
You can confirm with 'dpkg -l' that's a package in 'rc' status:
$ dpkg -l | grep libjack-jackd2-0
rc libjack-jackd2-0 1.9.6~dfsg.1-2 JACK Audio Connection Kit (libraries)
You may probably want to remove definitely those configuration files; just purge the package. For example:
$ dpkg -P libjack-jackd2-0
See also about debian configuration files, an important assumption Debian takes.