Server 2008 & Limiting Simultaneous VPN Connections per User

I am trying to implement a VPN (RAS) on a Server 2008 and need to limit each user to only 1 VPN connection. So far, I have found some articles, but run into errors during implementation. I found one software package that describes what I am looking for, but at a per-user cost. Ultimately, I would like to have a way for this to be self-managed within Windows without requiring any additional software. 

Has anyone successfully done this and/or have recommendations?

Links to articles:

http://www.ryanaverill.com/2012/12/limit-rras-to-one-concurrent-connection.html

http://blogs.technet.com/b/rrasblog/archive/2007/12/20/steps-to-develop-a-ras-administration-dll-usi...



Related Software:

http://www.isdecisions.com/products/userlock/