Go click Campaigns from the top menu. Click Edit on the campaign that you wish to deactivate. On the left-hand side, you'll see an Archive button. This will automatically deactivate your campaign.
Alternatively, you're welcome to archive a donation form, from the Campaigns page, by clicking the archive button:
You may reactivate a campaign any time by clicking Archived Campaigns located on your dashboard and hitting the Unarchive button.