Would utilising sysinternals TCPView tool to identify the process and terminate it outside of MSSQL not help with these SPIDS? Often with past experience and SQL versions, the SQL process had lost its handle to the operating system PID when this kind of thing had occured.

1) Download TCPView http://technet.microsoft.com/en-us/sysinternals/bb897437

2) Run TCPView, locate the connection to linked server and close the connection.