Block Double Logins protects your WordPress membership sites by preventing users from sharing accounts. It’ll save you time and money when dealing with dishonest customers.

plugin is ready for use 5 seconds after activation
block users from simultaneously using the same WP account
block users from simultaneously logging in to multiple accounts from the same IP
block users who hide behind a proxy
see who’s online at any given time
see who’s trying to share their account
choose which user roles are affected by plugin’s rules
built-in fail-safe in case you lock yourself out
session timeout can be adjusted
works with all themes