Hi,
You need to go for enhancement for restricting service order edition with validation if service entry sheet created for your service order.
Discuss with intelligent ABAPer and go for development with getting data from EKKO and EKBE tables. EKKO-EBELN with PSTYP(Item Category in Purchasing Document) = '9' and Pass the EKPO-EBELN as input & fetch all the required records from EKBE table with BEWTP (Purchase Order History Category) = ' D(Lerf)”. Now on above data, you can validate for restricting service order edition by enhancement
Regards,
Biju K