Which interface is used to sort wrapper class column?

hi all, 
when opp stage is "closed won" then none of the fields should be editable. i have created a validation rule for this.
AND( ISPICKVAL( StageName ,"Closed won"),OR(ISCHANGED( Type ),  ISCHANGED( Amount ), ISCHANGED( LeadSource ), ISCHANGED( CloseDate )) )

now when i change the stage field from qualification to closed won along with the changes in any other field. record should be saved but its not getting saved.
what mistake i am doing ? pls help.. do let me know is there anyother way to do this..Thanks