Well, you could set up your own login module. Consider this pseudocode:
1. Verify correct login info.
If incorrect - usual stuff, if correct,
2. Grab user IP from headers and check if username already exists in a table of some sort with a different IP associated with it.
If yes and time is within a specified interval (say 1 hour), show message and refuse login or ask for some other info to force the login.
If IP is not different or time interval is longer than specified, save username, IP and time and proceed with login routine.
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One of the problems that exist is that you have no way of forcing users to actually logoff. So, he/she can just close browser or simply walk away and create a possible issue with this login thing above.