Logging from another PC

Hi, I have a question. When the user logs using own username and password into Radius server (ie, using 192.168.160.5), it is OK. When someone change IP address statically into logged IP (to 192.168.160.5), he can use the logged account. I mean he can use another one's account. How can I block another PC? And I don't want the user logs often in one day. User must logs once in a day. That's why I don't want to put Idle-Timeout attribute. I'm using FreeRadius 2.0.1 with Cisco'BBSM 5.3. Could you give some clarfication for this? Thanks