Sub contract PO (Item Category "L" )

Dear Friend,

There is requirement from my client

In case of Sub contract Purchase order (Item category "L"), user should not have option to make any changes for "Child material" which is exploded through BOM. i.e. In PO Creation or Change transcation(ME21N or ME22N) at item level >Material Tab >Components option (which will show the details of "Child Material") should not appear or it can appear but in gray field.

Can any one help me to fix the issue

Thx

Deva