I believe that placing a "Reject" UD code on the lot will automatically block automatic payment and require accounts payable to manually intervene and remove the block. The SAP process is not based on the movement of material to blocked stock. You can often accept partial shipments and if you did how would the system know how much the debit note should be for? Should it be for the exact amount of the cost of the material moved to blocked stock? Or would you want to ask the vendor for "additional considerations"? I'd want additional considerations personally cause now I have to spend time sending this back or finding an in-house use for it.
If you really wanted to do this, you could use a follow-up action on the UD code that would look at the stock postings and determine what went to blocked stock and how much to make the debit note for. The problem with this is that you have to make sure the action doesn't run more than once or if it does, it can determine if a debit note was issued or not. I.e. what if you eventually find more bad stock? What if someone just changes the UD code for whatever reason? Is the follow-up action on all UD codes or just some?
FF