There are plenty of plugins that allow you turn your WordPress site into a membership service. To keep your business going, you will need to protect your premium content. Block Double Logins is one of many plugins that can help with that. It is a simple plugin that stops users from sharing their accounts, including simultaneous logins.
Block Double Logins allows you to see who is logged on at any time. It stops simultaneous logins from the same IP and visitors who hide their identify behind proxies. You have the option to adjust session timeout settings. It even has an override code just in case you lock yourself out.
This plugin simply protects your membership business by stopping people from sharing their accounts with others. WebFactory is the developer behind this.