If you would like to cancel a campaign that is currently running, then visit your Promote > Campaign List area, and click the down arrow to the right of the campaign you'd like to cancel. Choose "Cancel Campaign."
You can also "Cancel Renewal" to just cancel the upcoming renewal of a renewing campaign, and keep the current one running until it finishes. This information is also available in the User Menu > Settings > Services area.
Here is a video which shows the cancellation process: