One login at a time – security

I have an online tutorial site for secondary students, to access I charge a small fee. I have set up self registration, and wait until payment is received and then I verify the users.

Is there anything I can do to deter students from signing up and then sharing their passwords with all their mates to avoid payment?

I’ve considered installing the users online plugin – would that show that the same student was logged on more than one computer?

Can I block a user if they are already logged in? I have also seen wassup plugin that will show users IP addresses etc – this might be useful.

ANy ideas

Cheers