There is no way to keep a user from logging in a second time available in the QC/ALM UI. The following Site Parameters can be used to release an inactive connection after a period of time. The problem with limiting users to one login is that this limitatin would also mean that a user could not be logged into the QC/ALM client and use the Excel addin since it requires session to connect to the server.

I have previously opened a case with HPE regarding this issue and this was HPE response

HPE's response...

As commented, in this moment there is no functionality where we can restrict multiple logins for users.
However this is already reported to R&D and in this moment they are working on this and we might have this implemented in our new ALM version; ALM 14.

I also asked if this could be accomplished via workflow and received the following response from HPE...

I was checking with my backline team and unfortunately, there is no way to implement a workflow.
This happens because the workflow we can apply it is just inside the project and not out of it in where we can apply some code restricting the multiple logins.