You will find a delete button in the upper right corner of each customer record. This action completely removes the record and cannot be undone.
For data integrity and proper auditing, some items cannot be removed. You can only delete a customer if there are no associated projects, invoices or payments. If the action is not permitted, the button will be greyed out.
If you have not collected any payments from the customer yet, see Delete Project and Cancel Invoice to determine whether you can update other areas that will allow you to cancel the customer.