Restricting chargeability on a Task

Is there a standard way to restrict a Task from being charged for PO Requisition but still allow for AP Invoice (Some of the invoices are coming from external systems)

I thought of following options but doesnt seem to work:

1) Using Transaction control, but it will restrict for both PO and Invoice

I had similar question for Project. Client wants to restrict the Project for PO Requisitioning but still needs to receive an invoice against that project. Pending Close status of a Project works but it doesnt allow to create a non po invoice.

Is there any standard way of handling this.