I don't believe that there is a specific way to do what you are asking.
If you are running NetBIOS on your network then I know that there is an option in the user account to say that the user can only logon to a specific PC.
I've cheated - their home drive (includes profile settings such as desktop, My Docs, systems settings etc.) is shared individually - with a maximum of 1 connection.
We use ScriptLogic, which if it can't access the home directory will refuse logon. In a non-ScriptLogic enviroment (where you are just using Group Policies) then Windows would still allow logon, but refuse access to the desktop etc.
Maybe if you were to do this, but create a WSH Script, where when the user logs on it runs. The script tried to access a system file or something - but if the script can't (due to max connections) then it will log the user off automatically?!
Steve.