Limit PPTP connections per user

Hello,

Does anyone know if there's a way to limit the amount of
concurrent connections on a per-user basis? Basically, I
just want one user to only be able to have one VPN port
open at a time. For some reason, Windows allows a
seemingly unlimited amount of connections per user, by
default, and I'd like to change that. Thanks for your
help.

Regards,
Max Carper