Hi Nick,
ACS only provide authentication (and authorization and accounting) services with various ability like limiting LOGIN sessions to/through a device, controlling what command to execure (tacacs+) and so on.
However, I don't think ACS has the ability to limit how many sessions can be opened/established from a client to a server or application. This should be controlled by either by the servers or applications.
A device like firewall (PIX/ASA) can only limit connection to a server, but not from a device.
Rgds,
AK