These are a couple of our retention reports. As with most of our reports, these reports are available for CSV export.
Admin > Reporting > Inactive Accounts
This report lists customers that do not currently have any active series. By default, the report shows customers who have purchased a series in the past, but you can also look at just customers who have only ever been comped a series.
Admin > Reporting > Unused Accounts
This report lists customers that have never purchased or been comped/given a series.