How do I cancel an UPLOAD?

APPLE APERTURE: I decided to upload some photos to my PHOTOSTREAM. The process started okay but seemed to be taking a long time. I decided to cancel the task and upload later. To stop the task, I opened the 'ACTIVITY' window where the taks are listed. I selected the task and clicked CANCEL TASK but the upload will not stop.

I have tried closing the program and restarting - Activity is still there.

I have tried shutting the Mac down and restarting - The activity is still there.

Any advice on how I can stop/cancel activities would be gratefully received.

Thank you. 🙂