Introducing a router isn't something RADIUS/PPPoE/Hotspots can prevent. A NAT'ing router makes everything behind it appear as if all traffic is sourced from the router, so from your router's perspective there is only one client. You can use the firewall mangle facility to change the TTL of all packets going back to the customer to 1, if the customer is using a router that router will decrease the TTL to 0 and discard the packet. If the customer isn't using a router the packet will make it to the directly connected machine with a TTL of 1 and be processed just fine.
As far as I know User Manager does have an option for how many simultaneous logins an account can have, but that doesn't address routers masquerading devices behind it as explained above.