If you have a campaign that's currently in progress that you wish to stop please follow these few steps.
First find the campaign you wish to stop. Next check-mark the box on the left-hand side of the campaign title text and click clone.
This will make a clone of the original campaign and preserve the contents for the next step.
Find the original campaign and check-mark the box on the left-hand side and then click delete.
What this will do is erase the campaign and prevent it from sending out any further emails to the contact list.