check particular cell value and not insert again new value

Question

Hi,

    In my database I have 3 columns ie., Id,Name,Subject(Marks).I inserted records as 1,Sai, 20.

         When I am trying to insert new marks to the Sai ,it won't accept new marks to the Sai.

          That means I want to restrict to re-enter the marks for particular record and it allows only one time to enter marks..

           How to write code for this.. ??