There are a couple of ways to pause a campaign. From the Publisher and from the Advertiser.

Pausing from the Advertiser

1. From the left side menu, open the Advertiser and then the Campaign where the Campaign is assigned.
2. Choose the Campaign from the list on the right
3. Under Zone Assignments You will see a list of Zones where this campaign is assigned.
4. Under the Status column, you'll see a green Active. Click this button and it will change to a red Paused. Do this for all Zones you wish to pause this Campaign

Your campaign is now paused and will no longer be served.