After you save a draft, schedule, or send an email you'll be taken to the email's manage page.
To stop an email that's in the middle of sending, click the "Stop Sending This Email" on the top of the manage page. This will immediately stop sending your email.
Some emails will have gone out, however. You can now exclude the stopped email from future emails to make sure activists don't get an email twice when you send to the rest of your universe.
For example, let's say you made a mistake in your email and stopped it after 100 emails were sent out of a total of 1,000 to be sent. To correct your mistake and send your email to the remaining 900 activists, you'd copy the email you stopped, fix your mistake, and on the targeting create page exclude the email you stopped to make sure those first 100 activists don't receive a second copy of your email.