You have the option to Cancel a promotion completely, or you can just Pause it by setting it to Draft, while you make any changes needed.
To do this please log in to Jump Send and go to your Promotions page. From there, click on the Published button of the promotion you'd like to pause/cancel:
Once you're in the edit page for that promotion, scroll down the bottom and click on the 'Set as Draft' blue button...and also click the red Cancel Promotion button if you want to Cancel and take the promotion offline:
After that, your promotion will be unavailable to Jump Send shoppers.