VA02 modification( user exit )

Hi,

I have a requirement:

In the sales order if the item category is "ZLPK". Then

that item must be blocked for changes in the VA02, That is for that particular item all input fields in the VA02 must be input disabled.

Is there any standard way to do it or we must do it via user exit. Please help me how we can do it.

Thanks,

Santhosh