How to set a field inactive at selection screen?

Hello, here is the definition of my selection screen.

SELECTION-SCREEN BEGIN OF BLOCK b1 WITH FRAME TITLE text-001.

SELECT-OPTIONS : s_typcmd FOR EKKO-BSART DEFAULT text-003, "Type de commande

s_grpe FOR EKPO-MATKL ,

s_ebeln FOR EKPO-EBELN. "Numéro de commande

SELECTION-SCREEN END OF BLOCK b1.

I want to set s_grpe to inactive.thx