Can the approval user groups delete/reject a PO?

No, the approval groups cannot delete the PO. However, they can reject a PO. 

• Go to “Accounts”
• Click on “Purchasing”
• Click on “Purchases”
• Click on “Reject” next to the particular entry
• Enter the Reason for Deletion/Rejection (Rejection will trigger an email to PR creator with the appropriate reason provided) 
• Click on Submit/Reject PR.

Note: Only the admin can delete the PO.