Disable Screen-Input for checkbox in report selection screen.

I have defined one Selection Screen parameter as follows:

PARAMETERS: CCCDoc as CHECKBOX USER-COMMAND check DEFAULT 'X'.

Now, When Selection Screen comes up I want to have this check box greyed out. I tried this code but is not working for me. What differently should I be doing?

AT SELECTION-SCREEN OUTPUT.

LOOP AT SCREEN.

IF SCREEN-NAME = CCCDoc.

SCREEN-INPUT = '0'.

MODIFY SCREEN.

ENDIF.

ENDLOOP.

Please advise.

- Rohit.