Prevent unauthorized use of an outside user

[Capt'n Clax]

I run a private fishing forum with vbulletin 4 and have recently found out that one of my users has given his username and password out to an unauthorized person. Being that this is a private forum, i want to know if there is a way that I can snare this culprit. I don't know who the user is that has handed out his password. One thought I had was to search the IP addresses of all users and see if there is a suspicious pattern, and then block all IP address with the exception of the IP address they used to sign up. Once I dove into seeing everyones IP addresses, I realized that this might be futile since many of us look at the forum from so many computers. The second thought I had was to reset everyone's password. But, I doubt that would do anything to help my cause.

I'm not terribly proficient at vbulletin, but I'm a quick learner. Is there anyway to manage this situation????