Azure conditional access policies make this really simple and the following screenshots ill show how we can create this conditional policy.
Browse to the Azure Active Directory admin center / Azure Active Directory/ Conditional Policies
Next we assign what users the policy will be applied to
Select the cloud app - Exchange Online
Select the client app - Active Sync
Select the controls to enforce
Finally save & enable the policy
Now when a client attempts to setup and use the native Apple IOS app , this message will appear in the end users mailbox, the native app will be unusable for sending and receiving messages. The user can then proceed with the device enrollment process.