On 15.2 version, deleting a customer is not allowed.
Make sure that the Origin Integration on Company Preferences is disabled, otherwise the Delete button on Customer screen will be hidden. Refer to How to Disable Origin Integration.
From Accounts Receivable > Maintenance > Customers menu, open the Customer record you wish to delete.
Click the Delete toolbar button.
Select Yes button on the confirmation message. Note that you cannot delete a customer record that is already used in a transaction.