Campaigns can be paused and resumed anytime by switching back and forth from published to draft mode. Pausing a campaign will temporarily stop the actions from firing off in a campaign until it is republished.
Step 1: Click on the campaign you wish to pause.
Step 2: In the top right, click Published and toggle to Draft. This will pause the campaign. Anyone in the campaign will be listed as "Paused" under status.
(You can check this by clicking on the number of leads in a campaign, and in the middle column, under "Status", you'll see Paused.)
Step 3: To resume the campaign, repeat Steps 1 + 2, toggling from Draft to Published. The campaign will be active again.
NOTE: Those already in the campaign will be resumed at the campaign step they were paused at, and any time delays will remain. (For example, if they were paused after Campaign Step 1, and Campaign Step 2 is an email sent after a 60-minute delay, when the campaign is resumed, the 60-minute countdown will begin.)