You can do it from a custom module.
These are the files that control the form:
- \XLite\Model\Payment\Processor\PurchaseOrder class
- skins/default/en/checkout/purchase_order.tpl template
Most likely replacing the template file with a custom one that displays only a single PO field will be enough. You can do it either by making your module to be a
custom theme, or by
decorating the getInputTemplate() method in PurchaseOrder class and making it refer to your custom template file.