Typically you would find the document that the task is attached to in the Documents list and go to the Reload tab. From here you can disable the task.
However, there is a slight issue if you have deleted the document from the server before removing the task - as the document does not appear in order to allow you to remove the task. If you are in this situation create a dummy document with the same name as the old deleted document and place it in the location of the old document. After a refresh in the Management Console (F5) it should then be possible to remove associated tasks before removing the dummy document again.
In QV10 there is a link from the status page to edit the details of each task - however I have not attempted to see what that does if the target document is missing.
Hope that helps.