If you have created a subcontractor PO by enabling subcontractors on one of your job phases (and selecting the sub and contract value) you would have to navigate back to the Plan&Track section of the job in order to delete this PO.
First, open the job this subcontractor PO was added to and navigate down to the job phase in Plan&Track where it was added. You'll notice under the subcontractor heading, there will be a clickable link that lists the [PO#___] next to "original contract"
Clicking this link will open a window that shows the original PO amount and gives you the option of editing the subcontract:
After clicking edit, you will have the option to delete: