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

Pausing from the Publisher

1. From the left side menu, open the Publisher and then the Zone where the Campaign is assigned.
2. You will see a list of Assigned Campaigns to the right side of this list you will see a Status column.
3. From the list of campaigns, find the campaign you need to pause. To the right, under Status, you'll see a green Active. Click this button and it will change to a red Paused.

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