Hi,
Go to OPL8 for the order type you are creating production order in the tab Purchase requisitions maintain as 3 for "Reservation/Purch.Req." So as soon as your order is created your reservation number will be created. And the reservation number you can see in MB25. If you want reservation numbers to be created after releasing then maintain it as 2.
Also In MB25 you can tick the open reservation and execute u get the both type of reservation like manually or system generated (like production order) u can filter manual reservation using reservation status M and use transaction MBVR for only manage to delete the manual reservation.
Transaction MB25 is a display list. Reservations are not removed from it because you don't physically delete the reservation when you delete the production order. You mark it for deletion instead as well as mark the production order for deletion and the reservation still shows in MB25. In fact if you access one of those reservations you should see it with the deletion flag on.
On the contrary it would disappear from transaction MB26 which is the actual picking list.
Regards,
Narresh