Make quantity field of Free goods as non-editable.

Hie!

I have free goods scheme of 9 + 1 .If quantity of main material is 10 then it will split into 9 + 1 where 1 is free of cost.

While making the sales order the free goods quantity field is editable. So the users are able to change the free goods quantity. I want to make this field as non-editable.

Please suggest how to do this.

Thankyou.