Planned order - Exclude from Rescheduling

Hello Gurus..

We use planned independent requirements to get an understanding of what needs to be produced in the coming weeks. When MRP is run, it creates the planned orders to satisfy the independent requirements, which is fine. But when there is a shortage of material, system is trying to reschedule one of the planned order (for independent requirement) to satisfy the demand and the quantity is too high on the planned order, which is causing us to have stock at the end of day.

E.g.: If there is shortage for 100 each on 07/31/13 and there is a planned order for 08/02/13 for 5000 eaches (for independent req), then system is rescheduling the 5000 eaches to 07/31 so that it can cover 100 each shortage on 07/31. So end of the 07/31, we would have 4900 eaches sitting on the floor, which we dont want.

So i was thinking if we can exclude only the planned orders that are for independent requirements from rescheduling..

Please suggest how this can be achieved..

Also, in some cases when MRP reschedules a planned order or a purchase requisition with more than required qty, it causes us to have ending stock, which we don't want. How can we achieve zero ending stock every day?

thank you...