How to block input area? You can check below method.

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PARAMETERS: p_werks LIKE mseg-werks OBLIGATORY DEFAULT ‘PHAS’ MODIF ID bl2.
AT SELECTION-SCREEN OUTPUT.
LOOP AT SCREEN.
IF screen-name = ‘P_WERKS’ .
screen-input = ‘0′.
screen-output = ‘1′.
MODIFY SCREEN.
ENDIF.
ENDLOOP.