Use the Process class to get a list of the threads currently in the process. That will give you access to the thread id's.
Then you can use OpenThread API to obtain a handle to the thread, then use TerminateThread API to kill the thread.
Be sure to close the handle you open as well with CloseHandle.
pinvoke.net: openthread (kernel32)
pinvoke.net: TerminateThread (kernel32)
pinvoke.net: closehandle (kernel32)