Can s2Member restrict simultaneous logins?
Yes. s2Member's Simultaneous Login Monitoring will help you prevent a security breach.
Prevent Customers from Sharing their Username
As with any membership system, it is possible for one member to signup, and then share their username with someone else; or even post it online for the whole world to see. This is known as username sharing. It is not likely that you'll be attacked in this way, but it's not a bad idea to protect your system; just in case somebody tries this.
s2Member's Simultaneous Login Monitoring will help you prevent a security breach. The rules are simple. A single username can only have X number of simultaneous logins (configurable). Once that limit is reached, s2Member assumes there has been a security breach. At that time, s2Member will place a temporary ban (preventing access) and the offending username will be unable to login until somebody else (who is already logged into the account) has logged-out; clearing the way for someone new.
See also: Does s2Member track IP addresses?