Hi Leo
It is not advisable to delete any orders in SAP.Better to keep reason for rejection. Just click on item and see the status bar, you can able keep the reason for rejection. I am not able to recollect the exact screen as i am not infront of SAP.
Also check this note if you have any issues with reason for rejection in scheduling agreement.
http://www.stechno.net/sap-notes.html?view=sapnote&id=103750
Regards, Sai Krishna.