How to solve alive user who actually has loged off

[???]

Hello everybody,

I am using freeradius-1.0.4.

In my situation, user "testuser" from domain "domain" logged from NAS 192.168.1.68, 

After a while,user "testuser" logged out,but the NAS did not send Accout-Stop packet(for some reason) and freeradius still 

thought that user "testuser" is alive,but  Actually user "testuser" has logged off.

I have set the Simulate-Use to 1 and now "testuser" can not log in any more from anywhere.

I use radwho and it output as follows:

# ./radwho -R
User-Name = "testuser at domain"
Acct-Session-Id = "02022911"
NAS-IP-Address = 192.168.1.68
NAS-Port = 98335
Service-type = Login-User
Framed-IP-Address = 0.0.0.0
Acct-Session-Time = 4112
Calling-Station-Id = "00d0-5926-e3fb"

I want to do something to make freeradius to believe that user "testuser" is not alive,but do not konw how to do.Is there a way to solve the problem?
Thanks!