Need help to disable input selection-screen - very urgent

Hi SAP experts,

I have a requirement where in I need to disable ( Grey out ) the input fileds on the selection screen .

My problem is as I am using ABAP query I am not able to use any events ( AT SELECTION-SCREEN OUTPUT ).

My selection screen looks like this

MATERIAL NUMBER ( SELECT-OPTIONS)

MATERIAL TYPE ( PARAMETERS )

Now I want to add one more check box below the above fields on the selection-screen.When I click on the check-box,the MATERIAL NUMBER must be greyed out and MATERIAL TYPE should remain the same.

Please let me know the coding for the same. All answers would be rewarded.

Thanks in Advance,

Suresh.