Trying to delete all customers

Trying to delete all customers

We are having a problem with the customer list.  Some were imported from Quickbooks, while others were manually entered.  I can't tell which is which, but when I try to upload an expense report into Quickbooks, I get an error saying the Customer doesn't match.  Best scenario would be to completely delete all customers and reimport from QB; however, when I try to do that, I get an error from Zoho Expenses saying You have recorded transactions for this contact. Hence, this contact cannot be deleted.  I've gone through every single report and made sure a customer is not listed on any of them.  When I run the report for Expenses by Customer, nothing is listed.  I cannot figure out where this is coming from.  How can I delete these and start over with the customer list?  This is frustrating.  We changed to Zoho Expenses from Expensify last month, and I'm starting to think about going back to Expensify.  Thanks!