First of all, we have an ASP.NET forum, why did you not ask there ? Second, there's no way to do this. A new session is created when someone reconnects to your site, and so an IP address is your only option, and that won't work if someone has a dynamic IP and reconnects to the web. The only way for this level of control is to issue people with a login and let every login only work once.
Christian Graus Driven to the arms of OSX by Vista.