Can I stop users from sharing accounts?
 

I have a site with a subscription area, is there a way to only allow one computer to be logged in at a time on the site so one paying customer can't share the account with all his freeloading buddies?

You can try Multiple account login detector (AE Detector) .

That works the other way where it is trying to prevent people spamming by holding conversations with themselves... I just want to make sure when I give a professor a free account, he doesn't share it with the whole class when it was just for him to use.
Isn't there a good way of having the first session disconnected when the second is logged in?

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Is the problem that vB uses cookies to keep a user logged in rather than a string in the URL like my bank would? I am wondering if this could be something I could resolve in the paid help area, or a basic difference in functionality that it would never happen.