De acordo com este tópico em que um PFE MSFT entrou na conversa - você não pode.
Brian Day, um PFE do Exchange, diz:
You cannot disable it for everyone at this time. I would suggest asking the users to give it a good month of use, and give it an honest chance.
Conversation view may takes some time for some people to figure out, but the majority of the time people end up being far more productive with it. Lots of real life people studies went into observing how people were productivity-wise with and without it before making it the default view.