If you need to disable OWA access to all user In your organization In one go or In bulk, you can use Exchange Powershell.
Note: This process will work on Exchange Server 2016.
To disable OWA open Exchange managment Shell (EMS) and type:
Get-Mailbox -ResultSize unlimited |Set-CASMailbox -OWAEnabled $false
To enable it back to all users type:
Get-Mailbox -ResultSize unlimited |Set-CASMailbox -OWAEnabled $true
The change will happen immediately and will be disabled or enabled on all mailboxes.
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