I shall create a workflow approval workflow for purchase orders. Today, the approval routing feature enabled (in enable features) and approval routing for purchase orders in accounting preferences is disbaled. The supervisor of the employee is approving the PO, and there is a mail going out to the vendor when the PO is approved, with the approver as the sender. It is that email I want to make sure disappear. What is causing that email to go out? There are no customizations.

Not sure exactly what you want, can you explain it a bit clearly? Also what you mean by "Disappear"?