Destroying a Thread

This question is a follow-up to:
http://groups.google.com/group/comp.lang.java.programmer/browse_frm/thread/28d348f31e4d3ed7/#

I am using the BlockingQueue as suggested in my first post. It works
great (well, still not sure about a few things, but I'm trying to
figure them out). I did figure out that, if I want to stop the
thread, the best way is to return from the run method, so I have
something like this:

/**
 * @see java.lang.Runnable#run()
 */
public void run() {
  while (true) {
   try {
    File file = queue.take();

    // check to see if the queue has been poisoned
    if (file.equals(FileWorker.POISON_FILE)) {
     break;
    } else {
     this.processFile(file);
    }
   } catch (InterruptedException e) {
    e.printStackTrace();
   }
  }

  return;
}

That way, if I close down the project, I can put a File object in the
queue that lets the thread know that it is finished (AKA, the project
has closed). (I clear the queue first so that the poison object will
be the only object left in the queue.)

The problem I am having is this - if a process is already running, I
don't really have any way of interrupting it (which I want to do if
the user is closing down the project because I don't want the very
processor intensive process running in the background while they
continue to work).

Any suggestions for stopping a process? (This would also be helpful
because I want to allow the user to choose to cancel a process that
they have already started.)

Thanks