Allison,
Just like everyone is telling you, there isn't an easy way to do this. You
should really look at why you want to enforce such a policy and how that
fits into the business needs of your organization.
If you want to do a bit of coding, you can write a logon script that will
track the logon events in a database with a time stamp. If there is a
record for that user tied to a workstation then call the Logoff.exe file to
boot them out.
This isn't terribly hard and would work, though it is not using actual logon
as its information source. You would also have to put a script on logoff to
clear it and use the time stamp as a threshold -- in case they just
disconnect, so as not to lock them out forever.
There may be a way to check if there are logons programmatically, but I
don't know that part of the API. Maybe I'll get constructive tonight and
try to code this for you.