If you're an admin, you can deploy a .reg file to users' machines to turn off Coming Soon. To create the .reg file use the following steps.
Copy the following text into the file.
Save the file and change the extension from .txt to .reg.
Tip: To change the extension of a file in the Save dialog, put quotes around the file name, for example: "DisableComingSoon.reg".
Right-click the file you created in Step 3, and select Merge from the pop-up menu. This will create the correct registry key and set its value.
Note: You may have to click Yes to allow the Registry Editor to make changes to your computer.
If you use Group Policy to manage Office settings, there is a policy setting named "Disable Preview Place" that you can use to turn off Coming Soon. The policy setting is located under User Configuration\Policies\Administrative Templates\Microsoft Outlook 2016\Outlook Options\Other. To use this policy setting, download the latest Administrative Template files (ADMX/ADML) from the Microsoft Download Center.