Thanks. I already received a reply in Yammer. I changed the service item to flushing finish at the BOM and now I follow these steps:
1. Start PO
2. Picking list is generated without the service item
3. I book the picking list
4. I book receive and invoice at the purchase order
5. The purchase order receive generates the picking for the service item and book it automatically
6. I can finish the production and no futher picking lists are generated for the service item.