My guess would be enter a batch credit note for the O/s balance for each of those accounts and then do a Customer Receipt for each customer to clear to Zero. Then you should be able to mark them as inactive. It's a long winded process I'm afraid.

I don't suppose there's any chance of importing the batch credit note from a spreadsheet is there? Also would the customer receipt affect my bank account?

Hello Heather - You can import credits in as Audit Trail Transactions only - there are several links from a guide here 

By creating a credit note that is the same value as the o/s balance for the customer when you do the bank receipt it will net to zero so wont affect your bank recs.

Thank you!

The import file worked like a charm!

I was a bit thrown by the column looking for a Nominal Code but I put in the "Debtors" code and now the Debtors Control Account and the Debtors Account both have nil balances.

A bit late now, but you can use File / Maintenance / Rebuild

The Rebuild option (sage.com)

You can choose to remove all customers and all their transactions while keeping the rest of your database intact.

Use with extreme caution!