Campaigns can be ended when they are in an 'Active' state. Ending a campaign will cancel all future surveys and not allow any further feedback to come into your campaign.
You will still have access to all of the campaign data, but you can no longer send surveys or receive new feedback. Once the campaign has been ended, you'll see your campaign now in a 'Complete' status.
If a contact tries to fill out a survey when the campaign has been ended, they'll receive the message, "This campaign has now ended."