Hi,
Several years ago someone implemented a way to force a single session per user, it is disabled by default and I have not used it myself so I'm not 100% sure whether there are any side effects or problems enabling it, but you can give it a try by adding this to your user.config file:
<add key="ForceSingleSessionPerUser" value="true" />
then touch web.config to make it recycle the app and reload settings.
Hope that helps,
Joe