I need to prevent users from creating new email accounts in outlook 2013

I have found that some of my users have been setting up secondary email in their outlook such as gmail, yahoo, etc.  They are only authorized to use exchange email.  How can I prevent this from occurring?
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AmitIT ArchitectCommented:
What level of rights your users have on their machine. You can use GPO to restrict it.

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Will SzymkowskiSenior Solution ArchitectCommented:
The easiest and most effective way to control this is at the firewall level. You can do this for individuals or the entire organization. You will also be able to see what people are trying to attempt to make connections to their gmail using pop or imap etc.

denver218Author Commented:
ended up using GPO.  Thanks.
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