how to deny "sudo su"

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http://serverfault.com – I have several servers where some users require to be sudoers to work. The problem is that when sudoers can run the command sudo su and login as user root. It seems very risky to run that command. I tried with Command Alias ​​in the file /etc/sudoers but it has not worked. Is there any way that they are sudoers but not run the command sudo su? (HowTos)