In an organization you may not want to get prompts to try out the new Outlook Preview versions. Luckily, there's a way to turn this off, either through GPO or through a registry key, which can be distributed to your users.
If you're using the latest Administrative Templates for Office, you can find the setting under User Configuration\Policies\Administrative Templates\Microsoft Outlook 2016\Outlook Options\Other, where you can enable the Disable Preview Place setting.
Alternatively, add the following DWORD-value:
Note that the above only disables the user prompts and does not hide the "try new Outlook" toggle. Add the following registry key to your clients to hide the toggle as well: