Block simulteneuos logons

[jd () eyesizwe co za]

I need assistance with the following query.. My company has a lot of remote access users and the problem that I am 
having is that most of my users use their AD accounts simulteneously. What I mean is that my users have dialing access 
and dial back options and what they do is they allow their family members to use their accounts during the day while 
they are at work working. So what I want to do is to block or allow only single logon for all users on the network. I 
want my system to be able to detect if the user has already logged on be either by dialup or network and deny the 2nd 
logon. what solutions can I implement?