Do you sell access to premium content and are concerned about users sharing their account with others?
Simultaneous Login Killer (SiLK) automatically logs out users who use the same account at the same time, making it much harder to share access to your premium content.
And then you have the deactivation option. When things get out of hand you can first show a warning. If that's not enough, the system can automatically deactivate the user account (deactivate not delete)
The icons included with the add-on are from IconDock.com