Dear Aunt TUAW,
Ah, the problems of modern romance. Since I broke up with "her" (and yes, I know you were saying she wasn't right for me) getting rid of all the remants of the relationship has not been easy, especially "her" digital footprint.
She loved her music. So she would bring her iPod to my house and sync it leaving a whole bunch of songs I'll NEVER play again that were purchased under her Apple ID. How can I find them, delete them and finally be ready to move on with my life?
Your favorite but knucklehead nephew,
When in doubt, there's usually an AppleScript solution to everything. Doug's Scripts offers one called "track down purchases" that scans your library, looks at the file for each song, and retrieves the apID purchase account for the track. It then builds individual playlists that sorts the songs by account.
Be aware that you may encounter snags for homebrew recordings such as those from GarageBand, because the script expects to encounter standard iTunes tracks. If so, you may want to temporarily remove those while running the AppleScript.
Let Auntie know if that does the job for you.