How do I stop a campagin that is running?

You could remove the flow step for the e-mail to be sent. If I understand correctly, your campaign is set to Wait X amount of days and then send an e-mail if criteria isn't met. It won't read the email step until the Wait step is completed. 

To be safe, you could change settings in the Schedule tab to "Deactivate."

How do I know that it, for sure, will no longer send. If I change a program when it is running, I didn't think the changes would take?

The changes take and there is no save button. If you edit a flow, the changes happen immediately.
I get what you're saying, though - are people queued up to receive e-mails, or does it take everything one step at a time.

What I am saying is that I believe it takes things one step at a time. Especially if you deactivate the campaign, everything should stop.

I would want to delete the third send email (refer to picture attached)

If you're not completely deactivating the campaign, you can just remove the 5. Send Email step. (and the 4. Wait step).

Basically, people aren't queued up to receive the Send Email step. What happens is that the system only looks at the next step once they leave the Wait step.

Here is more on that idea from an actual Marketo employee: https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PGBLAA4

Happy campaigning--

Thanks.