Some days ago, Microsoft released a free app to access and manage (not only) Exchange-, Office 365-, Outlook.com-, Gmail- and IMAP-Accounts. This app was made by Acompli, a firm that was bought by Microsoft some weeks ago, and simply rebranded.
If you want to block this app from accessing your Office 365 Exchange Online, this is the way to do it.
From within you Office 365 Admin center go to 'Exchange admin center'. Then click in the left menu on 'mobile' (1) and then click on 'mobile device access' (2)
In the next step click on the plus-sign (+) under 'Device Access Rules'. Then set the values according to this screen shot.
Click 'save'. You are done.
If you have a Exchange on premise or if you want to use Powershell to block the app, this article will help you.