As a User you can stop a campaign created and scheduled in your User level account.
Admins can stop campaigns they have created and scheduled.

If for any reason you need to urgently stop a campaign while it is actively sending (processing), here's how:

• Start by navigating to your Campaigns menu.
• From the list of Current Campaigns, click on the campaign you would like to stop. 
• Within the campaign editor, if you see a status of Processing*, click the Stop button (1).

• Confirm that you would like to stop the campaign by clicking Stop on the alert (2).

It's important to note that if you do stop a campaign mid process, any sends that have already occurred cannot be recalled or cancelled, and the campaign will not be able to be re-started.

If you would like to send this campaign at a later date, you will need to duplicate it or create a new one.

* If the campaign status is Finished, it cannot be stopped, recalled or cancelled.