From your dashboard, click the "Edit" button for the campaign you want to automatically end and navigate to the "Initial setup" page.
In the 2nd panel, click the "Optional Settings" link.
Click on the "When do you want this campaign to end? (UTC)" field and pick the date you want to end your campaign.
Anyone trying to enter your campaign after the end date will see the following message after submitting the form.
"Hi there! This campaign has now ended. Thanks for checking."
Click "Save and Continue".