Hi
As righty said by Eduardo please use that userexit. You can also put restriction to delete line item using this or may be some other userexit but all exits are there in MV45AFZZ program. I remember I had done this using MV45AFZZ and we had to try in different exits one by one. I had the same requirement from my client 
and if I remember correctly we used USEREXIT_FIELD_MODIFICATION and
USEREXIT_MOVE_FIELD_TO_VBAP exits.
Thank$