You can pause any running campaign from the campaign screen directly. Just find the campaign you want and toggle the On/Off switch to off:
You can also pause a campaign from the Campaign Detail page.
Any messages that are scheduled to go out while the campaign is paused will be skipped. Those missed followups will not be rescheduled if you turn the campaign ON again, so we recommend only Pausing an entire campaign if you are positive you don't want to send any followups out.