You made a mistake and assigned a job to someone and now you want to cancel it. To cancel the job, if it appears in your ToDo list, simply use the drop-down box on the right hand side called "Job Options". At the bottom of this list of options is "Remove Job" as highlighted in the following image.

If the job that you assigned does not appear on your ToDo list, you may need to use the "View" drop down box and click on "My Jobs" as seen below.

The reason for this is, when a job is assigned and the first "task" in the "job" is assigned to someone else, it will not appear in the current task list until that person has "finished" that task.