Occasionally we all hit a button and then wish we could take it back! If you have submitted a request and wish to call it back, you can change the state of a request back to New, meaning that it has been retracted from the staffing department's domain. At that point, you (the PM) can edit the request, or delete it altogether.
To cancel a staffed allocation, right-click the allocation and choose Cancel Staffing Request from the context menu. The status of the allocation will change to New.