What is the command to "Block sign-in" on a user with Exchange online Powershell?
There is the GUI version of what I'm trying to accomplish. 
What is the command to "Block sign-in" on a user with Exchange online Powershell?
There is the GUI version of what I'm trying to accomplish. 
None - this is an Azure AD operation, not Exchange one. You can use Set-MsolUser (from the MSOnline module) or Set-AzureADUser (from the Azure AD module): https://docs.microsoft.com/en-us/powershell/module/azuread/set-azureaduser?view=azureadps-2.0#parameters
If you are using Exchange Online/Office 365, a free Azure AD instance is included. You can run the cmdlets mentioned above by downloading the corresponding module and logging in as an admin.
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