I've seen that before and I think one of the best ways to handle this stuff (I'm about to do it myself) it to keep some kind of token on a server and remove it when same user logs in. That will kick out previous user - this can be done with push notifications to OTHER users.
It will work like so:
Person1 logs in as "User1" and writes tag "Person1:User1"
Person 2 comes along and logs in as "User1" - while logging in - it will detect that Person1:User1 already logged in and will kick him out by sending push notification. Person1 in fact may be out already, but if not - he will get notification that someone with same login just connected and he is being pushed out.
This kind of stuff will be obvious for users and they will buy licenses if that is a main point :)