How to Disable field in Selection-screen

Hi All,

I want to disable Screen field in my seelction screen. Below is my Code, but it is not working.

I want to perform.

If RB1 = X

Then Parameter P_VBELN should get disabled. Please suggest, find below my code.

*....Selection Screen

selection-screen begin of block b1 with frame title text-001.

parameters: p_vbeln like vbak-vbeln.

selection-screen end of block b1.

selection-screen begin of block b2 with frame title text-002.

parameter: rb1 radiobutton group radi default 'X',

rb2 radiobutton group radi.

selection-screen end of block b2.

*....At Selection Screen

AT Selection-screen.

if p_vbeln = ''.

MESSAGE ID 'ZID' TYPE 'E' NUMBER '000'.

endif.

*....At selection screen output

AT Selection-screen output.

if rb1 = 'X'.

Loop at screen.

if screen = 'P_VBELN'.

screen-input = '0'.

endif.

modify screen.

endloop.

endif.

*....Start of selection

Start-of-Selection.

select *

from vbak into table itab1

where vbeln = p_vbeln.

if itab1 is initial.

select *

from vbap into table itab2

for all entries in itab1

where vbeln = itab1-vbeln.

endif.