Why can't I install this app completely?

I'm trying to uninstall an old version of iDrum (moving all associated files to trash and emptying it) except I don't think it ever really uninstalls. I know this because during a fresh install, it asks for a new authorization code. How can I get rid of it completely to re-install?

if it asks for an authorization code, then there is no code present, so it probably is completely gone? look in the installer, there is usually a small comment as to what additional files it installs besides the .app bundle ... it's usually some preferences stored somewhere in the Library or a "." file which is a hidden file