ScriptHere's a simple prevent users sharing sign-in information plugin for WordPress, it will block simultaneous sign-ins
How prevent multiple login with same login credentials in WordPress ?
Introduction : WordPress users can sign in to their account simultaneously from multiple locations with same username and password, lets take an example, suppose if you’re running a WordPress membership site in that case people sharing your login information and wanting more access to your site, then it will undoubtedly damage your interests and you know that WordPress has a minor security breach. You do not have to worry about this I will explain how resolves this issue (i.e. blocking simultaneous sign-ins).
Full explantion please visit : http://scripthere.com/how-prevent-multiple-login-with-same-login-credentials-in-wordpress/