how to kill a thread

below is a sample code

i read a list of machines and use ThreadPool.QueueUserWorkItem to thread
each machine using a timer. If the machine is not pingable, i wish to kill
the thread that checks the ping function, so that the next poll will not run
again.

Any advice ?

************************************************** *********************

Private Sub OnTimerElapsed(ByVal sender As Object, ByVal e As
System.timers.ElapsedEventArgs)

For Each computer In Me.machines

m_utl = New utility(m_kbhotfix, m_domain, m_ntid, m_pwd)

ThreadPool.QueueUserWorkItem(AddressOf m_utl.pollmachines, computer)

Next

end sub

public class utility
Public Sub pollmachines(ByVal state As Object)

computer = CType(state, String)

If Not pingmachine(computer) Then

'how to kill the thread that does the last
pollmachines

End If

end sub

public function pingmachine as boolean
pingmachine =false
end function

end class