Limiting # of Simultaneous Log-on Per User in Portal

Hi Experts,

For security reasons, we would like to limit the users to only 1 logon at a time to the portal.  Currently, the same user appears to able to log on multiple times to the portal at the same time (for example, using IE, FF, Chrome at the same time with the same user).

We are searching for a parameter in the Portal configuration we could set to do this. We are currently using NW7.31 SP 11.

Bruce