As long as no transactions have been posted to the account you will be able to delete using the following steps:
Click Sales Ledger/Purchase Ledger/Nominal Ledger then click Customers/Supplier/Trading Partner
Locate the account required then click the Customer tab/Supplier Tab/Trading Partner
Right click Modify Record
Right Click and select Delete Record.
Select Yes to confirm deletion.
Then go to the Details Tab for the Customer/Supplier
Right click Modify Record
Right Click and select Delete Record.
Select Yes to confirm deletion.