Restrict user to single logon

[PreviousPoster]

I am looking to restrict a user to a single logon. We have an internal web site that authenticates to Active Directory. Our users us this to search for documents. We have one external user account for an outside agency. They use this logon several times at once which is making us get close to our license limit of the software. Is there any way to restrict this?
Thanks
Joe

[PreviousPoster]

There is nothing that keeps track if a user is logged in or not so your only choice is to restrict this in the web-application...