The Database Owner (DBO) has complete access to all objects within the database. You cannot restrict this users. The same is true for members of the System Administrators fixed server role. Anyone in this role has complete access to all databases and database objects.

Your correct course of action should to limit the use of SysAdmin and DBO in your organization.

BTW, you could get the behavior you're looking for by creating an UPDATE/DELETE trigger that used user_name() to determine who the user is and ROLLBACK any operations not performed by specific individuals. BUT - I'll stand by my previous statement as the true, correct, direction.

-Mike