was this using a tool in arcmap? and it doesn't have a cancel button? or it doesn't respond?
This was on ArcGIS Online... I eventually removed the layer that it was creating for the watersheds in My Content and after a few minutes it got rid of the processing in my map. But I didn't see an obvious way to cancel or see what the progress was on the Analysis. It also appeared that I could not Save my map while it was running.