Canceling lines on a Partially sourced Requisition

Our procurement dept would like to cancel two lines on a requisition where the first line has been approved and sourced but the second and third lines show as Pending /awaiting approval status. We tried cancelling the lines using the Requester's Workbench and Manage Requisitions menus but can't because the first line is already sourced.

When using the Manage Requisitions page: We clicked the Cancellation 'x' at the end of the lines, the status changed to cancelled but when we re-entered the requisition the lines' status was back to pending.

Can you please tell us the best practices for cancelling lines of a requisition when it is partially sourced?