How to indicate to warehouse and item has no stock and is a back order that should not be picked

Hello!

We get orders where we don't have stock.  We need to keep those items on the order as a back order after it has shipped.

Out picking slips show the item for which we know during order entry there is no stock. 

Is there any way for the back office to indicate to the warehouse that we know the item will be a back order, do not try and pick the item?