If the list had a changelog active since before the "some time in the past" you're interested in, the subscribe/add signoff/delete activity would be listed in there. Changelog reports are available via the web interface.
To unsubscribe from the LSTOWN-L list, click the following link:
http://peach.ease.lsoft.com/scripts/wa-PEACH.exe?SUBED1=LSTOWN-L&A=1