In my Team , we have a server which few people know the root passwordI can see they log into root and then usesu - user1to use from user1 login., I cannot change root password for some reasons , all i want to do is disabling then to do su - user1 at the same time , all other users does not need to follow this., they can be logged from root from su - 
Disabling su -  alone
password;su