Hi
Two things I have to share with you. First one is that after this warning message I don;t think so system will allow you to save sales document. If we block customer in XD05 you won't be able to issue any material, post any payment or any other document where financials are involved cannot be saved. Second one is that if still you need to give some error message on service order then you can take help of your ABAPER and convert warning Message no. IX063 to an error message.
Check and update.
Thank$