Block yum update

Hi all,

just recently someone accidentally ran yum update on incorrect host. It went fine but ... what if it didn't ....
Do you know any way to block just "yum update" (except changing yum python script) so no one would be able to run it unless it is unblocked ?
In the meantime I would like to be able to use yum with other subcommands but not the "update".
Any ideas ?