Hi
What do you mean by restricting material for service sale order and delivery order? First thing is if this is service then why do you want to make delivery for it? Second thing is if you want to restrict one material for this transaction then you can use many approaches and see which is suitable for you.
Extend/create one material only for that sale area which you will use for service.
Allow one material only and block all other using Distribution-chain-specific material status field in Sales organization 1 data.
Create a variant in SHD0 and make material field in grey mode and with content.
Use userexit and hard code the material.
Thank$