Set up a validation rule like:
AND( ISPICKVAL(PRIORVALUE(StageName), 'Closed Won'), ($Profile.Name <> 'System Administrator') )
After an opportunity is closed won, only system administrators can edit the record. But $profile.name seems to be not working.
Requirement: Once a record is saved as 'Closed Won'. Next time any user whose profile is not system administrator should not be able to edit the record