How to Log Out User Sessions in SharePoint Online?

Requirement: Force Log Off all sessions of a user, as he lost his mobile device which is logged-in with SharePoint Online sites.

How to sign out of SharePoint Online?
As an end-user, you can sign out from SharePoint by clicking on the User icon in the welcome menu, and then click on "Sign out" link. 

Once you are signed out, you'll see a message "You're signed out of Office 365".

PowerShell to Log Out User from All Sessions in SharePoint Online
We can force logoff a user from all his sessions opened with SharePoint Online using Revoke-SPUserSession cmdlet. Here is how:

#Connect to SharePoint Online
Connect-SPOService -URL https://crescent-admin.sharepoint.com -credential (Get-Credential)

#Logoff User
Revoke-SPOUserSession -User [email protected] -Confirm:$false 

This prompts for credentials to connect to SharePoint Online admin center and disconnects all sessions of a given user and takes them to login page.