How do I stop a Rank Tracker report from automatically running?

You have the choice of setting up your report to run on a weekly or monthly basis (scheduled), or you can elect for a report to run on an 'ad hoc' basis.

If your report is running on a schedule, it will automatically run on the date / day that you originally selected — to change this to 'ad hoc', simply go to the 'Edit Settings' section, then amend the scheduling accordingly.