Stop-TSSession

I'm trying to write a script that'll find all TS sessions our administrators have disconnected from and kill them. But I can't seem to get stop-tssession to work properly. Am I missing something obvious?

Thanks,
Greg

> Get-TSSession -ComputerName corphv14 -State Disconnected -Username *admin

Server          SessionID State           IPAddress       ClientName      WindowStationName UserName           
------          --------- -----           ---------       ----------      ----------------- --------           
corphv14        2         Disconnected                                                      CORP\mmadmin       

> Get-TSSession -ComputerName corphv14 -State Disconnected -Username *admin | Stop-TSSession -Force
> Get-TSSession -ComputerName corphv14 -State Disconnected -Username *admin

Server          SessionID State           IPAddress       ClientName      WindowStationName UserName           
------          --------- -----           ---------       ----------      ----------------- --------           
corphv14        2         Disconnected                                                      CORP\mmadmin