Office 365 batch user deletes

The Problem

You need to remove all (or many) synchronized users from an Office 365 tenant.  Maybe there was a mistake in scoping or you need to remove a domain that is associated with many User Principle Names (UPN).

The "Solution"

This can be done by a PowerShell script using the Azure AD commandlets: Get-MSOLUser and Remove-MSOLUser.  The catch is it's VERY slow.  I'm my testing about 3000 users an hour.  If you have 100,000 users, this will take a long time.  At it's simplest the below will work

Connect-MSOLService

Get-MSOLUser -synchronized -all | where {$_.userprincipalname -ne "yourUserId@yourtenant.onmicrosoft.com"} | Remove-MSOLUser -Force

Get-MSOLUser -ReturnDeletedUsers | Remove-MSOLUser -RemoveFromRecycleBin -Force

As of 2016-02-13, there is no better (or faster) way to do this according to Microsoft Support.