User exit / BadI to manipulate deliveries that get CIF'ed to APO

Hi experts

Here is a scenario where i need some help.

We have standard subcontracting scenario and we use ME20 to create deliveries to subcon vendor . We then do a goods issue to issue the product to the subcon vendor.

I want to suppress deliveries to subcon vendor from showing up in the planning book. The reason for this requirement is the demand gets duplicated if the deliveries to subcon vendor show up in the planning book and we order lot more than we want.  I must somehow be able to differentiate deliveries to customer from deliveries to subcon vendor . How do  i do this. Is there any user exit or BadI associated with sales order CIF where i can add custom logic to identify the deliveries created to subcon vendor.

My idea is to check if the delivery is to a subcon vendor and if so  assign a custom category to that delivery and exclude that category from the category group , so that the delivery wont show up in the planning book. Please help

Thanks

Saradha