Hi,
You can modify SCREEN, dynamically,in your ABAP program during the PBO event of a screen. Its contents override the static attributes of the screen fields for a single screen call.
http://help.sap.com/saphelp_nw04/helpdata/en/9f/dbab6f35c111d1829f0000e829fbfe/content.htm
For this you need to write the below code in one of the MODULEs in PBO event (1000 screen of your program ).
LOOP AT SCREEN.
* you can specify IF conditions here according to your screen elements
screen-input = 0. " 0 will disable accepting input ; 1 will enable accepting input
MODIFY SCREEN.
ENDLOOP.
This link will be helpful to you:
http://help.sap.com/saphelp_nw04/helpdata/en/9f/dbab6f35c111d1829f0000e829fbfe/content.htm
