I have ssh set up so my users can only access sftp and port forwarding, how can I limit the number of concurrent logins on a per user basis?

In my sshd_config I have UsePAM set to yes and in /etc/security/limits.conf I have:

username - maxlogins 1

I also tried:

username hard maxlogins 1

Neither of these works and the users can still log in multiple times.