The easiest way to block legacy authentication across your entire organization is by configuring a Conditional Access policy that applies specifically to legacy authentication clients and blocks access. When assigning users and applications to the policy, make sure to exclude users and service accounts that still need to sign in using legacy authentication. When choosing the cloud apps in which to apply this policy, select All cloud apps, targeted apps such as Office 365 (recommended) or at a minimum, Office 365 Exchange Online. Configure the client apps condition by selecting Exchange ActiveSync clients and Other clients. To block access for these client apps, configure the access controls to Block access.
