How to delete External Users from all site collection in a single command?

We can remove the user from PowerShell commands as well

https://support.microsoft.com/en-us/help/3026385/how-to-remove-users-from-sharepoint-online

All other subscriptions must use the SharePoint Online Management Shell.

Note This option doesn't apply to Office 365 Small Business (P) organizations.

For more information about how to use the SharePoint Online Management Shell, go to the following Microsoft website:

Introduction to the SharePoint Online Management Shell

Start the SharePoint Online Management Shell.

Type the following cmdlet:

$cred = Get-Credential

In the Windows PowerShell Credential required dialog box, type your site collection admin account and password, and then click OK.

Connect to SharePoint Online, and then type the following cmdlet:

Connect-SPOService -Url https://<contoso>-admin.sharepoint.com -Credential $cred

Remove the user from each site collection by using the following cmdlet:

$ExtUser = Get-SPOExternalUser -filter someone@example.com

Note Replace the someone@example.com placeholder with the account for your scenario).

Type the following cmdlet:

Remove-SPOExternalUser -UniqueIDs @($ExtUser.UniqueId)

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About Yogendra Bhardwaj

Yogendra has been in the IT services industry for almost 10 years and Microsoft certified Professional. He specialized in Service and Delivery management, Design and development of Microsoft technology based systems including Collaboration, also having experience of Automation and integration of different platform. He has got extensive experience with Microsoft technology stake for Office 365, SharePoint 2010/2013, and .net , C#. Also having hands on knowledge on Telerik Sitefinity, site core, Visual studio Test Manager, He has extensive experience for App Dev and App Support projects with supporting end to end SDLC process for Support and Maintenance projects.