Bob, an Administrator at XYZ was furious when he discovered that his buddy Trent, has...

First you should encrypt the data passed between the parties; in particular the session key. This technique is widely relied-upon by web-based banks and other e-commerce services, because it completely prevents sniffing-style attacks. However, it could still be possible to perform some other kind of session hijack. By configuring the appropriate spoof rules you prevent the attacker from using the same IP address as the victim as thus you can implement secondary check to see that the IP does not change in the middle of the session.