Sorry for the delay, I missed the reply - if you want to prevent users from concurrently logging in from multiple locations, I believe you will need to look into your Identity Provider (IdP) settings. Mobile Services does not handle authorization itself but delegates to whatever is configured as the Cloud Platform account IdP, and therefore there is nothing much you can do from the app/backend/Mobile Services. The same applies to other cloud services as well.
Alternatively, and I guess that is not what you are doing, if you are using some backend-based login (e.g. plain old user table), you could maintain a flag tracking sessions for the user, and then add some backend logic to prevent multi-logins from happening.