Hi Kent,
Because of the way QBO works, it is not actually possible to delete a customer. They are simply made inactive and marked as "(deleted)".
The only way I can think to fix this would be to figure out the RecordIDs for the Customers you imported and use the Import/Export Tool to mass-update them all, changing their names to something like "Deleted 0001", "Deleted 0002", etc. and making them all inactive. You could then re-import your original list as Leads, because their names would no longer be present in the table.
- Justin