Deleting the workflow open instances

Hi,

I have the below scenario, where user creates a PO and WF triggers and waiting for the approver to approve it. Before approver approves it another user releases the PO manually. In this scenario, when the PO is released, the WF instance which is in In Process status should be completed and the WI for the approval should disappear from the approvers inbox.

Could anyone tell me how to achieve this.

Thanks,

MK.