Indeed, PortalKeeper will do the job perfectly. Yet the free version won't allow you to. You need to purchase the full version. We'll provide support to set up the right rules and get the desired behavior.
 That sounds great! Is it possible to get a trial of the full version?
Hi, sorry for the delay. We don't provide trials for the full version and I've got nobody available at the moment. If you get the full version (Gold membership on this site), we'll give you a hand to configure it and get the desired behavior. If you're not satisfied, we'll refund your money. Please let me know if that sounds good to you.
By the way, this exact behavior - prevent simultaneous connections with one user account - has been configured with PortalKeeper for a client of ours.